Think of it as a cross between a La-5FN and a Spit9.
The speeds are pretty much comparable to the La-5FN at almost all alt ranges, and being famed as a fighter with one of the most favorable thrust-weight ratio in WW2, its acceleration might even compare with the La-7.
The N1K2 is actually one of the most fast accelerating planes in AH2, between 0~250mph range - this characteristic of JP planes are often overlooked due to their low top speed, but the Ki-84 will finally be a JP plane that has enough top speed to consider the acceleration factor as a serious threat.
I'm not sure of its maneuverability status, but IIRC Mitsu commented that it would be on par with the Spit9, and I'll take his word for it. If we assume that is a optimistic vision it's still pretty impressive - the maneuverability in my own estimation, will be somewhere between the Spit9 and the La5FN/Yak-9U.
The armament, compared to the N1K2, would be probably pretty much weak - two 12.7mm machine guns and two 20mm cannons.
However, the Ho-5 cannons were derived from the Browning M2 50cal design, and its firing rate and accuracy would be impressive. The Ho-5 is mounted on the Ki-61-I - pretty reliable cannon IMO.
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Compared to the MA where speed is king, it is still about one notch too slow than most of the main contendors(such as the La-7 or the P-51D, Fw190D-9).
However, if the Ki-84 would really accelerate that fast, then it would mean in most cases the Ki-84 would reach higher speeds more quickly than other planes. This is of very great importance in combat - especially low-mid alt combat which is dominant in the MA.
Also, any plane that has some balance between maneuverability and firepower is a lethal plane. In AH1 this was the reason why the Spit9 and the N1K2 was so popular. Eventually, with the coming of AH2, realistic gunnery ruled out the Spit9 and the N1K2 as an influential plane, since they can't lob 500yard shots so easily anymore. The Spit9 and the N1K2 was simply just too slow for AH2.
Now, imagine that the Spit9 receives some upgrades - the power and acceleration of the Lavochkin fighters. A Spit9 that is far faster than it used to be. That, is IMO undeniably a serious threat for many.
Even more profound, is that the Ki-84 can wield combat flaps. A fowler type flap with "combat" position that merely extends straight backwards to increase wing area, but add very low drag.
The fowler flap(coupled with no torque!) is what makes the heavy and titanic sized(its actually bigger than the P-47) P-38L so maneuverable in AH2. Imagine the same efficient flap type that is used by a much more lighter, nimble, single-engined fighter.
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Like someone mentioned, the Ki-84 will probably become the last plane that holds enough importance to potentially transform MA plane choices and usage numbers. If my estimates are right, the former Spit9 and N1K2 croud will all move over to the Ki-84, and it will form a new "Big4" with the La-7, Fw190D-9, P-51D. I predict about 8~12% usage rates constantly.
It is definately not perk material - with a plane like the La-7 and the P-51D, Fw190D-9 free, frankly no prop plane is perk material(at least, high perk material ranging over 10 perks) in the MA. However, it is a plane with a potential that might bring some alternative fighting styles in the MA.
