You broke a wing off your F4U going 500? Didn't think that was possible
485 is about the top a Ki84 can go before the ailerons rip off.
Here's thing thing, if you have a KI84 diving on you, and you know he's going 450+. All you gotta do is cut throttle and do a high bank BFM turn to get outa the way. They won't be able to slow down quick enough or be able to pull the nose up as they are compressing. Edit: ONLY DO A HIGH BANK TURN IF THEY ARE COMPRESSING. you'll be dead meat if they aren't. If they aren't, you'll want to do a hard nose down turn or horizontal turn to give them a hard lead shot to make. You can use that to go into a barrel roll defense, but that is a bit more advanced move that takes a lot of practice and timing, albeit you should definitely learn it if you want to be really successful in defense.
Also, if you see them start to pull up as you are diving from compressing. Once they get about 800 above you, you can do a sharp emmilmann and pull up into them and you might get a shot. It's a move that works well against Ki43s. If you miss though, be prepared to nose down and fly away from them.