Actually the Ki100 was supposed to have been equipped with a 1,500Hp Ha-150 engine- a home-grown development of the DB series designed specifically for use in a new interceptor. When the factory that was to build the engine was destrtoyed in a bombing raid, it was decided instead to mate a Mistubishi Ha-112-II engine to the Ki-61-II-KAI airframe. The engine mounting was, incidentally, based on the Fw190A.
Performance was slightly slower than the Tony (366 MPH at 36,000 feet), but the engine was much more reliable (and probably more resistant to damage) and the the aircraft was much more maneuverable than its predacessor. Its ceiling was also slightly lower (36,000 as opposed to 38,000 feet). The Ki100 did perform better than the Tony at altitude however (Ki61 reached its top performance at 15,000 feet).
Both carried the same armement packages, and the Ki100 had about twice the range.