Wrong on all counts. Ki-100 = Ki61. Ki-84 is better in every category, including guns, ammo load, roll rate, manunverability (one of the most manuverable planes in the game) and climb/vertical manuvering.
Ki-84 is the best Japanese plane in the planeset, currently. Adding a copy of the Ki-61 won't change that.
I do have a bit of an issue with the "all controls pop off at 450mph" feature, though. I don't think that's accurate.