Ki100 is almost identical in performance to Ki61. Same weapons, same performance, acceleration, top speed.
It was not re-engined to get more performance. It was re-engined because they had completed airframes but the engines were bombed to hell. They stuck a radial on with some modification to the cowling around it, and viola, production moves along again.
Only has minor differences to the speed and power curves up at about 18k or so. In some areas even worse than the Ki61 due to drag.
All the constantly perpetuated stories about it being super, being a uber bomber killer, about killing 14 hellcats on its first engagement, etc, are all lies, fabrications, and urban myths that have been debunked by folks in the know.
Kind of like having the Fw190A4 and the Fw190A5. What's the difference? About 6 inches longer nose, and nothing else.