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Methinks you have been misinformed.
The Ki-100 and N1K2-J were two of the best Japanese fighters of the war.
According to the WWII combat pilots who have given talks at the Planes of Fame, the best four Japanese fighters of the war, in no particular order, were:
1) Mitsubishi J2M Raiden "Jack"
2) Kawasaki Ki-100
3) Kawanishi NiK2-J Shiden-Kai "George"
4) Nakajim Ki-84 Hayate "Frank"
The founder of the Planes of Fame, Ed Maloney, a world-renowned authority on Japanese types, agrees. The Japanese themselves compared the Ki-100 against the Ki-84 and came to the conclusion that the Ki-100, if flown by the same pilot, would always win the engagement. As I'm sure you already know, the Ki-100 was a radial engine variant of the Ki-61 "Tony".
Of course, opinions, like your gas mileage, may vary ...