It appears that the late war japanese fighters ,could not compete with the late war allaeid fighters . Despite their powerful engines and their small and light airframes, they were 40-50 mph slower than P51D with the super duper fuel , and over 80mph slower than the P51H
The late war Japanese fighters (Hayate, Raiden, Shinden-kai) were 20-30 mph slower than P-51D (430-440 mph vs. 410-427 mph). Their engines were powerful (when able to be operated in full power), but the engines were radial types, and that costs in drag, despite efforts to streamline the installation. The engines were also single stage designs, and that shows at 25000 ft and above, where Merlin Mustang can use the thiner air to outperform much of other fighters.
Even at the rate of climb , despite their better power and wing loadings , they were not even close to the american designs eg
Ki 84 with 1,8 kgr/hp and 172 kgr/m2 had a roc of 3790 ft/min
F8F1 with 2,07 kgr/hp and 192 kgr /m2 had a roc of 4570 ft/m ! And with water injection would reach 7000ft/min !!! Double that of ki84!
Ki-84 with water injection was good for 4275 ft/min at SL, the F8F-
2 (=1948 plane) making 4300 ft/min when using water injection (2500 HP) at SL.
I know , americans had better propellers, wings of very very low drag and very very high lift, two stage superchargers and gyroscopic gunsights .
Bearcat's wing was not that very very low drag item - 23000 series wing profile, 18% thick at root, 9% at tip. Lift was probably a major plus, though. Bearcat never received two stage engine, the E series engine in the -2 was a performer, never the less.
It appears that despite their best efforts , japanese were at least 2 years behind in fighters performance.( Or even 5 years behind if the alleid claims ,from the china front, that P40s were superior to the ki 84, are true)
Talking about P-40 being superior than Ki-84 is silly (not sure what you're aiming at??). Japanese were rather late in introduction of 'second generation' of ww2 fighters, maybe a year behind the allies?
Generally, my personnal experience from studying WW2 aviation books. is that alleid aircrafts needed much less power to achieve superior performance in comparison with the axis aircrafts, and less stuctural weight to achive similar or superior structural strength.
Again, not sure what you're aiming at?
Some Allied planes were fast for installed power size/weight (P-51 vs Spitfire and Bf-109, A-20 vs. Beaufighter, Mosquito vs. pretty much anything similar), other ones were not that fast despite power (Typhoon, Hellcat, Corsair) - weight, size drag being detriments. Some were slowish despite the small size (Yak-1, war-production MiGs, P-40). Some were great under 15000 ft (Typhoon, Tempest, P-39, Allsion Mustangs, much of the Soviet fighters), other were no good under 20-25000 ft (P-47). US stuff was heavy, UK Soviet ones were lighter. US stuff was again considered rugged, usually carrying greater weight of fuel/ammo/weapons (not saying punch was greater, just the weight of weapon ammo).