As a fighter, I used to think the Ki-84 was superior to the N1K, but after flying them more I've changed my mind. I believe the N1K is superior because it doesn't have all of the weaknesses listed above. You lose a little bit of speed, but the ability to dive without fear of losing parts is a big plus, and the control surfaces remain effective past 400mph ias. The truckload of 20mm doesn't hurt, either. 
I've never felt very threatened by a Niki in my Ki-84; unless my SA slips and I don't spot them until they are 400 off my 6 and opening up. The Ki-84 is a step above the Niki. Once you've engaged and moved the fight into the verticle the Frank will quickly gain an advantage when your speed hits the magic number to drop flaps.
As far as high speed manuevering goes you need to disable combat trim. Similar to the P38, the Ki84's combat trim will fight you at higher speeds. If your coming around in a low yo yo with some real juice but feel that no matter how hard you pull you aren't getting close to riding the black out you should just try giving it a bit of elevator trim. That will pull you right into your maximum turn.
I haven't lost parts on my Ki84 in a dive for a year at least now. You can certainly dive a bit but if the p51 or hog wants to get away and has some air beneath them you should go ahead and slowly level out. You can follow them a ways and if they decided to come back up to play you've got all the advantages you need.
The way I think about it is the Ki84 is the complete opposite of the P47. In a P47 you want the fight to be constantly going downward, in the Frank you want it to be nose up as much as possible.
As far as the OP goes I tend to think that a lot of the playerbase really has no idea of what a Ki84 is or is capable of. I'll routinely catch all but the fastest of the planeset; the Ki84 has a great top speed on the deck. I'll destroy spixtweenies in the verticle or in a turning match. Often p51's only option is to dive and run and if they don't have enough E stored to get away before I hit critical speeds they are in trouble.
If the Ki84 wasn't modeled with inferiour construction and crappy japanese gasoline it would have to be perked. Especially the 4x20mm version. :drool: mmmmmmmmmm