No, it was A6M2 that performed better than P-39D and P-40E at altitude. That was clearly indicated by combat over Australia, Dutch East Indies and New Guinea in 1942. The supercharger enabled it to engage targets at 20,000 feet without problems , and up to 30,000 feet if it was really necessary.
Here is a quote from "Eagles of the Southern Sky" book ( Eagles of the Southern Sky ) :
"After the war, former RAAF No. 75 Squadron pilot Pete Masters would recall his time fighting the Tainan Ku. On the performance of Zeros he wrote : There was much more we know now that we didn't know before we got to New Guinea, like the ability of Zeros to outperform us, contrary to what we had learned before we arrived. We were told that Japanese Zero was an upgraded copy of the Harvard Trainer as used by the Empire Air Scheme in Canada. It was underpowered and flimsy with very light aluminium and canvas cladding sometimes joined by canvas at the extremities. It had no armour plate to protect the pilot, and although it was reputed to fly higher than the Kittyhawk it was much slower and inferior in almost all its characteristics, including firepower. I am now sure that all this misinformation came about because very few Allied fighters had encountered Zeros in combat prior to New Guinea and those that had in Malaysia, the Phiilippines or elsewhere had never had a chance to examine the real specifications of this amazing aircraft at close quarters.
In fact when I at last saw a Zero on the ground and crawled all over it I too was amazed at the rigidity of its structure and its apparent fighting qualities from the cockpit. In our combat assessments at Port Moresby during the 44 days we fought, we soon concluded that head-to-head combat between Kittyhawk and Zero was unwise, and at altitudes above 20,000 feet was simply dangerous.
They were much more maneuverable and could outclimb us in the ratio of two or three to on. What they didn't have was our weight and pilot protection with 1/2 inch armour plate behind the pilot and in the firewall. Our battle plans therefore always included a desire to get above the enemy, if possible into the sun, and then to break away and come back for another shot. If we had height we could always get away from the Zero but with equal skills as pilots on both sides, the Zero always had the advantage at the same height and also could maneuver much more effectively above 20,000 feet where the Kittyhawk would tend to "fall out of the sky"."
Sakae 12 still maintained over 800 HP at 20,000 feet which is as much as on Sea Level at rated power. That's quite good actually.
Exactly same thing can be said about mentioned by you aircraft, P-40B had no protection and instead of 6 .50 caliber machine guns it had only 2 in fuselage and 4 .30 cals in wings. Was 1.500 lbs lighters as well. P-40B still was climbing slower than A6M2 and its speed decreased fast after 15,000 feet. Allison engine was not delivering enough power at those altitudes.
P-39C also had no protection for pilot or self sealing fuel tanks, it did not have two additional .30 cals added in D model. And climb performance was marginally better if compared to A6M2.
So not sure what makes them ... better ?
In regard to weight of the armament, I think its rather a positive feature that gun is both reliable and light. No designer wants a lot of added weight.
You mean once it acquired three more 13.2 mm Machine guns, bulletproof windscreen, armored plate behind pilot seat, underwing racks to carry air-to-air rockets and 250 kg bomb ? Obviously. But thats also because Sakae 31 was not performing as expected with water&methanol injection and engine was reversed to a similar performance of Sakae 21.
It only restored the performance ... to a level of 1943 A6M5. The aircraft reached 563 km/h which is marginally slower than A6M5 which had a top speed of 565 km/h, the time to altitude was improved by mere 11 seconds. However landing speed has increased, thus indicating also a higher stall speed. Aircraft definitely had inferior flight characteristics, including the maneuverability.
We already had that discussion how brilliant it was be to fit A6M with Kinsei engine. Except it was not ...
This statement is absurd.
In 1940 the Mitsubishi received from the Kaigun Koku Honbu Gijutsubu the preliminary specifications for the 16-Shi Carrier fighter which was intended to replace the Zero. It was intended that the new aircraft, which would not be available in quantity for several years, would become the Navy's standard carrier fighter when the Zero had reached its limit in modification and improvement.
However, in 1940 the Mitsubishi was experiencing serious difficulties in the development of the 14-Shi land-based interceptor (J2M Raiden); the company was plagued by a shortage of competent engineering personnel and the lack of a satisfactory engine for the J2M Raiden design. These circumstances delayed the 16-Shi carrier fighter program for at least a year.
But despite eventually solving problems with J2M the development was again postponed. In the autumn of 1941 Horikoshi became seriously ill and did not recover until several months had passed. The Navy preferred not to undertake the design project without my active participation. First engineering meeting to plan the new fighter was held on 14th April 1942, at the Japanese Naval Air Research and Development Center. In the long interval which had elapsed, the plane was renamed the 17-Shi carrier fighter.
From Horikoshi memoirs on A7M :
The failure in providing a successor of A7M was than a combination of bad luck and stubborn Navy, Kaigun Koku Honbu Gijutsubu forced on Horikoshi a decision to use NK9K which proved unable to deliver expected power and effectively making aircraft underpowered. Horikoshi from the very beginning tried to lobby for much better developed Mitsubishi MK9A which not only delivered more power but was also very reliable and could be maintained by even less experienced ground crews. His opinions were rejected until flight tests carried on A7M1 in June 1944 proved that Homare engine is not a good choice here and during a meeting in late July decision to use MK9A was taken.
A7M2 first flew in October 1944 and all tests indicated that aircraft finally had very good performance and characteristics. It could be finally put into production.
But then on December 7th 1944 a heavy earthquake in Oe-machi where Mitsubishi plant was located damaged the factory sufficiently to prevent any production there. 5 days later B-29s bombed Oe Airframe Works of the Mitsubishi in Nagoya temporarily suspended all production activities, and few days later B-29s made their first attack against the Daiko Engine Works of Mitsubishi in Nagoya and, on 18th December, returned to batter the Oe Airframe Works. The air raids smashed machines and production lines, killed hundreds of workers, and threw the great factories into a wild state of confusion. The New Year brought even worse air attacks; the Daiko Engine Works received the heaviest blows. It was hugely unfortunate for the Japanese that this tremendous engine plant was so badly hit, for it was responsible for the production of the Mitsubishi MK9A engine. At the same time Jiro Horikoshi fell sick, and development was continued Yoshitoshi Sone. But it wasn't until end of the war when Mitsubishi could start delivering A7M2.
The failure in providing a next generation fighter was as mentioned a combination of bad luck, stubborn Navy leaders and Mitsubishi lacking manpower to work on multiple projects. It had nothing to do with other manufacturers, who had their own designs to work on. No Air force is based only on fighters and Navy obviously had to call for other types of aircraft.
Sidenote here is that Horikoshi claimed later on that Navy would choose from the very beginning MK9A than fighters could be delivered before B-29 seriously started bombing Japanese mainland.
Which is exactly when ? Kinsei 40-series was rejected during development by Horikoshi himself. Series 50 became available in extremely limited numbers in 1942.
This is not correct either. There was never a choice between Sakae and Kinsei. There was choice between Kinsei and Zuisei.
And Horikoshi explained :
Source: Eagles of the Mitsubishi - The Story of Zero fighter by Jiro Horikoshi
Aircraft is always a compromise, and engine being a heart of the warbird affects all other systems - heavier and larger engine requires stronger and larger airframe, rises the amount of fuel that has to be carried to maintain the range, etc. etc.
And no, first prototype did not fail to meet the required speed. During first flight trials on 17 and 18 April 1939 aircraft reached a speed slightly over 490 km/h. After corrections (as you all know speed indicated by gauge in cockpit is not a true airspeed) the actual speed was recorded as 508 km/h. Requirement called for 500 km/h at 4000 meters.
Thus Mitsubishi satisfied the requirement during very first trials.
Sakae 12 was not installed by May :
Kinsei 40 series was rejected. Kinsei 50 series delivered a lot more power but consumption of fuel was also much greater. And at that time range was crucial due to combat at Salomon Islands (hence why A6M3 model 22 was developed). So potentially yes, aircraft could be provided with protection, but that would reduce drastically its ability to escort bombers at ranges they had to operate.
No. Japanese did not fell into that trap. Japanese did something else, their policy was to do a great jumps with every following machine - > A4N -> A5M -> A6M -> A7M. But problem between A6M and A7M was that Mitsubishi was doing exactly what you suggested, spending a lot of time designing further improvements to A6M and developing J2M. Then extremly bad choice for an engine, problems with Horikoshi health and lack of personnel slowed down any progress.