Here's my take on the Ki-84:
First, I like it very much and have had very good results with it.
Flown wisely, you can turn fight with anything short of the Zekes, and still give the A6M5 driver a major dose of angst because the Hayate is vastly better in the pure vertical.
I prefer the Ki-84 to the N1K2, which it will dominate at low speeds.
Beware of high speed dives, trim manually and exploit its terrific roll rate. Its gun package is good if not great.
I find the P-38J and Ki-84 a very good match with pilot skill being the determining factor. However, be advised that between 175 and 250 mph, any P-38 model out-turns the Ki-84. At any speed, the P-38G can turn with it. However, the Ki-84 can take the fight vertical at low speeds and gain the advantage. Not so against the P-38J or L. However, the Ki-84 has an edge in low speed turning against those. Just remember this; in a pure vertical climb, the Ki-84 WILL NOT get away from a Co-E P-38G. This type of climb is a zoom climb, and the P-38G zoom climbs better than the Ki-84 due to its greater mass. REN and I discovered this dueling a few months ago.
As for Spitfires, the Ki-84 is better in the vertical and can out-turn the SpitIX. I'd give a slight edge to the SpitV in pure flat turns, but the Ki-84 is far superior in the vertical, which easily offsets this.
Acceleration is good, but not spectacular. Climb is very good until that short WEP cycle becomes a factor.
All in all, I feel very secure when flying the Ki-84 as it one of the best non-perked fighters in the plane set.
It's not an easy plane to fly effectively, requiring trimming and close monitoring at high speeds. That is why the N1K2 is more popular. However, the more skilled the pilot, the more likely he will take the Ki-84 over the Niki.
My regards,
Widewing