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I don't think that the performance of the Ki-100 was exceptional. I think that the reason that it was well regarded by IJA pilots was that its engines were more reliable than those of the Ki-84, which was much faster, and much more reliable than those of the Ki-61-II-kai, which was also slightly faster. The undercarriage of the Ki-84 could also give its pilots unpleasant surprises.
If we ask how fast the Ki-100 was, we end in the normal problems with Japanese aircraft. There is a thread here at http://www.ww2aircraft.net/forum/flight-test-data/ki-100-peformance-30338.html and another at J-aircraft Ki 100 Performance. My take would be that the Wikipedia speed of 580 km/h (313 kn, 360 mph) at 6,000 m (19,700 ft) would be at military power. The Ha-112-II used water/alcohol for take off and gave 1500 hp against a military 1350 hp at 2000 m. Unfortunately, I don't know if the supercharger could give you higher boost at around 6000 m. If it could, then a WEP speed should be available for a short time.
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It climbed about like most Allied fighters or close enough for climb not to be a major factor, was more maneuverable than most, and had decent armament.
The two biggest weaknesses were top speed and lack of a good high-atitude supercharger.
But within it's decent altitude range it was quite good, according to everything I have read about it anyway. While there may be some, I have seen zero bad writeups on the Ki-100 by Allied or Japanese pilots. You can't say that about many Japanese fighters.
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While the level speed was never eye opening, the aerobatic performance of the Ki-100 was something of a revelation, according to the few pilot accounts around.
Several translated accounts I have seen label it as the best Japanese aircraft of the war. There are a number of contributing factors to this.
The aircraft was apparently a delight to fly, across the speed and altitude range. It could be effortlessly aerobatted, with light controls and very little directional instability. This made it a good gun platform and also useful for high speed attacks against B-29s at high altitudes.
The engine and its installation were considered reliable - a not to be underestimated point for the Japanese from early 1945 onwards.
The performance - somwhere between 360 mph and 380 mph best speed at altitude - was not drastically inferior to the fastest Japanese fighters and was close to the most numerous US opponent, the F6F.
The horizontal turn performance and sustained climb performance - two qualities highly valued by Japanese aviators - were probably better than anything in the Japanese arsenal. In a level turning fight or slow speed climbing fight, the Ki-100 was judged as always superior to a Ki-84, capable of handling two Ki-84s and able to work a 1 vs 3 situation to its favour.
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On the other hand it climbed better than its pregenitors, and in comparison with a captured P-51C it showed better diving characteristics. As most other Japanese fighters it also was considered more manouverable. But i Guess we all know by now that manouverable is a quite loose term.
Turbo-supercharged prototypes were flown in the hope of improving performance over 7000 meters. It's questionable how long it would have taken to get these to function reliably.
It was with most Japanese fighters, only 34 of the high attitude J2M5 was produced, and I have no knowledge how well the 3 stage supercharger of that worked.
At a guess, I'd think it would be somewhat like the P-36/Hawk 75 compared to the Spitfire I or 109E, with slower dive acceleration but better control characteristics at high speed.Claim that Ki 100 was a better diver than P-51C should raise an eyebrow - maybe you could shed some light on that?
I'd also would ask you to share some good information on turboed Ki 100, also on the 3-stage superchargers for the Kasei.
There is a caveat for the last paragraph, though - the US was, in 1945, using the P-51B-K, P-47D/N, P-38L, F4U-1 -4 - all of those put together make much more than Hellcat alone, and present adversaries that have from 30-90 mph advantage vs. the Ki-100, even if we assume the 380 mph mark as real.
I agree that those fighters have significant performance advantages over the F6F-3/-5. I don't agree that all these fighters combined were more likely to run into a Ki-100.
If you look at the aerial claims in theatres against the Japanese, there are some interesting statistics as to which aircraft actually shot down the most Japanese planes.
The F6F made its combat debut in September 1943. Between then and the end of the war, it claimed either 5156 or 5168 Japanese aircraft in aerial combat (stats differ a little) with the USN and the Marine Corps.
From September 1943, all other USAAF and USN fighter aircraft claimed a combined total of 5624 Japanese aircraft in aerial combat.
So, by my possibly dodgy math, that means that from the time it entered combat to the end of the Pacific war, the F6F accounted for 48% of all aerial claims by fighters against the Japanese.
The Bf 109 shared some of those same problems throughout the war ... the emergency fighter program in Germany was a bad (proposed) solution to a larger set of problems that had present to some degree from the start of the war with their problematic pilot training infrastructure and Bf 109 as the mainstay fighter. (the aging 109 airframe with its limitations -even heavily refined into the 109F- in range/endurance, cockpit visibility, and handling -especially on the ground)True but it points to what both the Japanese and Germans needed late in the war. Not just better performing fighters but fighters that didn't require highly skilled pilots to use them. Planes that were forgiving in both landing and flight characteristics and yet had enough performance not to be sitting ducks for the enemy. A 360-380mph airplane that was easy to fly beats the heck out of both a 330mph airplane that is super easy to fly or a 410mph plane that was tricky and unforgiving to fly and land.