This discussion rides on confusion, which concerns the question I asked earlier on.. so what is the perk agenda? Are the perks assigned according to:
 1) arena balance
 2) performance balance
 3) or, historical rarity??
  
 ....
 The C-Hog seems to be 1) (20~30% of kills), a bit of 2) (four Hispanos + carrier-craft + supreme JABO), and 3)... 
 the Ta-152 seems to be nothing but teeney bit of 2) (supreme speed at supreme altitudes where nobody flies..), and 3)... 
 
 Tempest would be a mix of 2) and 1)...
 What would the Ki-84 fall under then? 2) and 1)? ...
 But, the suggested performance data on the Ki-84 only seems to present to us that it is something like a bit faster accelerating Spitfire Mk9 or, a more maneuverable La-5FN... perhaps a Mix of a Spit9 maneuverability/top speed with La-5FN low-alt flight characteristics...?  
 Would that combination of performance be enough to make people choose the Ki-84 over everything else, and make it necessary of being a perk aircraft?? In this late-war MA environment where La-7s, Fw190D-9s, P-51Ds and Bf109G-10s roam??
 I really don't think so...
 We'll probably see them very often, but considering the rarity of Zeros(utterly incompetent as a fighter in the MA environment, if not rare..) and Ki-61s, it would probably be a result of fans devoted to Ki-84, or Japanese pilots who finally got their hands on a reasonably formiddable plane.. rather than something that has to do with initially great performance(as in the case of the Tempest)..  
 I'm willing to bet that with the Ki-84 introduced, a part of the Spitfire fans would shift to the Ki-84, some of the N1K2 loving folk would also go there...  but it won't in anyway need perks both performanc-wise, or balance-wise..
 ...
 Now, what JB42 suggests, I think, is an entirely different matter, perhaps, almost irrelevant to current discussion. It's about a totally different perk agenda.. almost.. 
  
ps) But I do agree along some of the lines JB42 suggests.
 I would like to see late-war aircrafts perked very lightly, (even more so than the C-hog... around 3~4 perks..). 
 P-51D, P-47D-30, Bf109G-10, La-7, Yak-9U, N1K2-J, Fw190D-9, F4U-1D, Typhoon 1B...
 which would promote the use of each predecessors
 P-51B, P-47D-11 and D-25, Bf109G-2 and G-6, La-5FN, Yak-9T, Ki-61, Fw190A-8 and A-5, F4U-1.. pushing the arena into a more "mid 1943~early 1944"-ish environment than now. 
 ... but, this is for another discussion..