Engine that powered Hellcat prototype was the two-stage supercharged R-2600, making plenty of power above 15000-20000 ft. Still it was not enough, the R-2800 was quickly installed in order to bring the performance close to the level required. US have had 100/130 PN fuel (in Europe also the 150 PN from early 1944 on), and used water-alcohol injection on most of the fighter engines. Japanese hi-oct fuel was 92 octane, helped in late war with w-a injection.
The XP-42 was never a combat-capable fighter. Vanguard looked like a fine fighter, eye-pleasing, thin-winged, but it was too late to matter with it's 1939-ish engine power, performance, protection and range.
The two Japanese aircforces were defeated by light engines, and by light and heavy AAA. Hellcat and Corsair mostly encountered target practice.
The XP-42 was never a combat-capable fighter. Vanguard looked like a fine fighter, eye-pleasing, thin-winged, but it was too late to matter with it's 1939-ish engine power, performance, protection and range.
The two Japanese aircforces were defeated by light engines, and by light and heavy AAA. Hellcat and Corsair mostly encountered target practice.