I'd recommend the KI84 if you are looking for a competitive late war Japanese plane. Fight uphill with it and when you get to that magic number for your flaps drop those puppies out and watch other planes flounder around you as you easily turn into them and saddle up for the kill. The only way they will get away is a prolonged dive and at that point you can just follow along with your altitude and wait to see if they want to try and climb back up to you.