This covers a lot of area/time.
Actually the P-40Ls were produced from Jan 1943 to April of 1943.
Allison was not producing enough more powerful engines. The 1943 engines had about 4,000ft more altitude than the 1942 engines. Change over was actually at the end of Nov 1942 with the P-40M model. But 15,000-15,500 altitude for an 1150hp engine wasn't good enough for a general purpose fighter in 1943. It was good enough to fill in and it was good for low altitude work.
The P-40 could be improved but it was going to take a lot of work and it wasn't going to be as good as the P-51B.
P-40 Myths
Most people confuse single-speed supercharger with no supercharger. I know you do not suffer from this misconception I just mention it because it is one of the typical sticking points.
The other myth is that it was slow and unmaneuverable but rugged (it was faster than almost every Japanese fighter it faced, and could out turn all of the German and Italian monoplane fighters). It's real flaws were that it climbed poorly and had a low effective ceiling. I.e. it didn't perform at anywhere near high altitudes.
In spite of this, P-40s were well liked by most (not all) of their pilots because once they learned it's strengths and weaknesses, it had a pretty high pilot survival rate. This was partly because it was indeed rugged, but ruggedness in a military aircraft is far less conducive to pilot survival than maneuverability. In the case of the P-40 this meant a good roll and turn, high dive speed and dive acceleration, and ability to continue to turn and roll at high speeds. The latter meant it was often able to disengage from combat when it wasn't going well, both in the Pacific and in the Med and Russia.
Australian Ace Clive Caldwell before his heavily damaged Tomahawk, just after a close call with a German Experten
An American pilot with the 79th FG in Italy examines a 20mm shell hole in his P-40L. He had 5 cannon strikes on this aircraft, and in the video this image is from he found the plug from one of the shells stuck in his wing. He wore that plug as a pendant for the rest of the war.
Early P-40s in US use were restricted to lower altitudes than most people realize
Allison-engined P-40s ranged widely in terms of their functional combat ceiling, which was a bit above the critical altitude for the engine. It varied mainly based on the supercharger gears. The typical altitude mentioned is 15,000 or 16,000 ft, but that was for the early Tomahawk variants, which (aside from Pearl Harbor) were really only used by the British and the Soviets. The P-40E, the one almost always featured in synopsis, movies, video games, and models, actually had a lower 'performance altitude' of about 12,000 ft. It was also slower than typically reported, at least at the original power settings, topping out a bit over 340 mph. They could be used at higher altitudes,
as they did successfully during the defense of Darwin up to 25,000 ft, but the aircraft is so starved for power at that height they are really taking a big risk, and that was only done out of desperation.
Later the up-engined P-40Es and K (which is just an improved E) got the Allison 1710-73 engine, which (which using higher octane fuel) made them faster especially down low (and conferring the oft quoted top speed of ~360 mph at their highest altitude), and yet the performance altitude was still about 12,000 ft. At 16,000 these planes were already struggling. This meant in the Med they were contending with German fighters that could dive down from 8-12,000 feet above them.
The P-40M and some marks of the N had a different supercharger gear ratio and pushed the critical altitude up to about 16,000 ft, and the performance limit to about maybe 18,000 ft. This was the 'hot' interceptor configuration of the P-40N, which included the first few hundred and some from later marks, which could make ~380 mph, but these were comparatively rare. By the time the N model came out Allison P-40s were only being used as fighters in the Pacific and Burma, and by the Russians. In the MTO the British were using them exclusively as fighter bombers and max speed was about 345 mph.
But because it was still always a single speed supercharger, there was no real fix here, as P-40 marks with the higher gear ratio (and thus higher altitude limits) had much less power at low altitude. Whereas P-40K and later model P-40E could produce up to 1570 hp at WEP down around 2,000 ft or lower (from the manual, not counting overboosting) which made them very effective at escaping pursuit in a diving escape maneuver, (and very dangerous to tangle with down low) - the higher geared Allisons couldn't produce much more than 1,200 hp at any altitude. So many pilots preferred the K. There were more Aces flying the K in China, the Solomons and in Russia than any other P-40 type. Later in the war (1944) the N and M ended up being used in India to cover the 'hump' air corridor into Burma because they could make it over the mountains better.
Restored merlin P-40 "Lee's Hope" at Duxford.
P-40F with Merlin was a major improvement
By comparison, the P-40F (available from late 1941) and L was much more versatile, because it had the two speed supercharger. So it had good speed at low-medium altitude speed, around 4 -6,000 ft, and then the higher middle altitude, with a critical altitude about 17,500 ft and good performance up to around 21-22,000 ft. (370 mph at 20,400 ft
according to this British test) This was of crucial importance in the MTO, which is why the Merlin-engined version was the only type used by the American fighter squadrons in that Theater (with a few brief exceptions of some K models due to temporary shortages). German and Italian fighters often attacked from as high as 25,000 ft and the high altitude performance of the Merlin Engined P-40s did well.
All five of the US P-40 fighter groups (33rd, 57th, 79th, 324th, and 325th), plus one independent squadron, (99th FS / Tuskeegee) active in the MTO had good to excellent combat records, and this is verified in the daily combat accounts in Mediterranean Air War. For a brief period from mid-1942, the P-40F was in heavy demand, until the middle of 1943, and arguably it was the best Allied fighter in the MTO overall until the Spit IX and later VIII arrived.
As for being a year behind, I don't think the P-40 let alone the F was ever really behind in the Pacific or China (it wasn't necessarily ahead either but when you look at realtively little known units like the 23rd FG in China and 49th FG in the Pacific they did very well). There was a general lag in the MTO where it was used against German and Italian fighters, in the sense that all nations seemed to take a while to send their most modern types down there, and it was as comparable to the Bf 109F and early G models that it faced, as well as the MC 200, 202 and 205, and the Spit V it flew with. The P-38 was probably a little bit better overall but it was having trouble in that Theater. I think the P-40F does deserve a bit more attention than it gets. It's one of those cases where a single type (P-40) has such a wide variance in subtypes in terms of it's effectiveness and combat record, that this model and the equally important K model tend to get overlooked. If it had a different designation it would probably be more famous.
They made the right decision with the Merlins - P-51 (and Spitfire) were better regardless
P-51 however was a better candidate for the 2 stage merlins both because of it's cooling configuration and because it was such an aerodynamically efficient / low drag design. Even with the Allison Engine the P-51 was 30-40 mph faster and it had the potential for much greater range. The P-38 and P-47 were also better than a P-40 once they worked out the various issues. The P-40Q was an interesting design which might have had a small role if not for the crash of several prototypes and overall major problems with Curtis Aircraft which put them on the outs with War Dept. officials, but I think the P-51 was still better. The Spitfire VIII and IX were also far and away better fighters than any mark of P-40.