Please, Please! Do not "perk" the Ki84!
Just because it can arguably be considered the best Japanese a/c flown in WWII should not make it a candidate for perk status. Compare its performance figures to some of the other a/c proposed as perks - it falls well below the Spit XIV, Tempest, F4U-4, P-47M, P-51H and Ta-152 or Me262.
The Japanese planeset is pitifully small in AH. I would much rather see the Ki61, Ki84, Ki100, and J2M3 added and perk the Niki if you must have a Japanese perk. The N1K2 did not see nearly as much combat as the Ki84 did. The Japanese also need a level bomber and carrier strike aircraft.
After some consideration, I don't agree with the "every country must have a perk" proposal. I would rather see perk status conferred by performance alone. The Japanese just didn't produce an aircraft that could compete with the proposed German, US, and British perk a/c, so any Japanese perk is bound to sit collecting dust in the hangar while the perk pilots choose the hotter rides offered by other countries.