Perks are applied according to arena balance in terms of usage. When a certain plane is hugely overused, it becomes a perk material.
However, since overuse is usually associated with MA-competitiveness, it is therefore indirectly related to performance; if a certain plane is very powerful in terms of performance, it will be perked.
However, the irony is, most of the perked planes never went through any kind of 'test phase' until they got perked. Almost all of the perked planes were introduced as a perk plane right from the start, and their prices have hardly changed. The Spit14 is a prime example of a misapplied perk. This plane fits none of the existing perk critera. It is neither a rare plane nor a plane that is anymore 'menacing' than the planes we already have free.
The same would apply to the Ki-84. It is a very competitive plane no doubt, and its wonderful balance between speed and maneuverability will make it easily one of the most popular planes in the MA. However, we already have planes like that. The La-7 is a Tempest class plane, the P-51D is easily the best multi-role fighter in the MA, the Fw190D-9 is one of the most powerful planes since AH2, and the Bf109G-10 is virtually untouchable at alt.
With that kind of competition, perking the Ki-84 is incredibly unfair. Personallly I say we perk all the late-war section of fighters lightly, between 3~10 perks. However, if none of the MA favorites are ever going to be perked, then the Ki-84 should also be never perked.(and the Spit14 too).