A good bird! I've been flying the Ki-84 for a good portion of the past two months.
As posted above, don't get going that fast, your aierlons come right off and your wings not much later.
Looking at loadouts, I recomend 50% fuel if you'll be staying close to base, add the droptanks for longer flights. I often carry the 2 250kg bombs and in that case I take 75% fuel; plenty to put your eggs on target and loiter for some kills.
Get in close for the cannons. They aren't hizooka's, but you do get plenty of ammo. Nailing a wing or tail in a snapshot at 200 with a few rounds will do the trick.
If you manage your E right and get worse turners trying to follow you can really put the hurt on them by forcing an overshoot and then rolling in behind.
I don't fear niki's near as much as I do in other planes. spits aren't terrible either. If you get any advice on how to beat a decent f4u pilot in one i'd like to hear, they give me all kinds of trouble if they have any alt on me, which is often the case.
I wouldn't recomend taking the Ki-84 very high, lots of stuff outclasses it up in the strato