Medium altitude is where the Ki-84 really performs well. By medium alt, I mean between 10k and 15k. Up there, you are not dealing with control stiffening and aero loading as this is a function of IAS (indicated air speed), not TAS (true airspeed). The lower it gets, the more control stiffening and aero loading becomes an issue. Control stiffening can be overcome by trim. Aero loading cannot be overcome, except by paying close attention to your speed.
Where the Ki-84 is at greatest risk is when speeds are just below corner velocity. At a speed near that, most American fighters can get one or more notches (for lack of a better term) of flaps out. This enables them to cut the corner and pull lead. Inasmuch as the Ki-84 can't begin deploying flaps until dangerously slow (danger in that it is a sitting duck for the third man in), its pilot must get the fight into the vertical and use the Ki-84's strengths to break down the opposition's E, where it can dominate the fight by forcing the fight up hill.
My regards,
Widewing