The lack of top speed isn't problem, high speed handling of KI84 is.
F4U, F6F, F4F, FM2, P38, P47, they'll all turn inside you if you come into fight with anything above 250mph. Not even trim will help you. Before speed drops you'll be dead against any good pilot.
To win you have to merge with 250 max and go first turn with zero throttle, drop flaps and you'll have your kill. Throttle management is very important.
If they have significant E adv, sticks like Shane will try to rope you, but often, good climb abilities of KI84 helps you to follow up. If you're less than 600yds bellow when he reverses on top, he won't be able to get angle on you on the way down.
Also watch the dead zone between 10 and 14k alt. MAN pressure drops there and you won't have the performance to outclimb the faster bandit.