Well, I've been flying the Ki-84 for about a week now, so I figured I'd share what I think about it.
In general, it performs similarly to the Spit 9. Both are relatively slow planes with good acceleration, climb rate, and low speed manueverability.
The Ki-84 has an almost unmatched ability to fly and fight at very low airspeeds. This is particularly noticable against other "turny" fighters such as Spitfires, Hurricanes, and N1k2's. The Ki-84 is almost as manueverable as a A6M at low speeds, and in fact may handle the vertical better.
In a 1v1 fight against a Spit 9 or N1K2, the Ki-84 definately has the advantage. The only caveat is the beginning of the fight- the Ki-84 is quite stiff at speeds above 300 mph, and you will be unable to pull enough G to "black out". Once the fight slows to below ~175 mph, the Ki-84 can drop combat flaps, and you should do so. These add a great deal to the stability and turning ability at low speeds.
In my opinion, the Ki-84 can hold its own against any plane in a slow speed "knife fight". Against Spitfires and Hurricanes (particularly the Spit 5), you have to be very careful because they can almost match your slow speed manueverability, and one hit from the cannons usually equals a lost fight.
The Ki-84 is one of only a few planes that can maintain a given altitude advantage practically indefinatlely against any number of planes. You can use this to your advantage if facing X number of planes... stay above them and go into a spiral climb when one tries to come up after you. He will stall out before you will, no matter what plane he is in. Judging when the plane is about to "flop over" is crucial for success in this kind of fight, but that is something you will have to develop on your own. If, indeed, anyone even cares about that sort of thing now.
Fast planes with timid pilots are almost impossible to kill in a Ki-84, the high speed handling sucks, so even if you manage to make them panic and blow their initial altitude advantage, all they have to do is dive, quarter roll, and pull out into a different direction. Above about 250 mph, you can't follow them.
The Ki-84 has a disturbing tendency to shed parts in a fast dive. For this reason, you should ALWAYS chop throttle in a steep dive. I mean always. The Ki-84 has very good acceleration, and above ~450 mph parts will start flying off. It takes very little time to get to 450 mph in a dive in a Ki-84.
Anyway, if anyone has any questions about tactics vs specific planes I'll check back and try to answer them, but if you think of the Ki-84 as a Spit 9 with better low speed handling, worse high speed handling, and half the firepower you'll do just fine.
