How much does altitude performance come into play in the MA's. Really not that much. Not saying that it isn't important, only that it isn't necessarily relevant. And the Ki-84 is right up there with the K4 as one of the best fighters in the game.
As for the - 1, yes it has poorer performance as far as climb and acceleration, but down low, it's pretty fast. It's also more maneuverable, and just a more flexible fighter all around.
IIRC, you've said other non quantifiable factors go into ENY, and I think the same should be true of perk price. Beyond that, are the P-51, K4, Ta 152, Spit 16, B-24, and others truly perk worthy? I don't feel they are, as they have to be flown by an ace, or at high altitude in order for their superiority to become apparent.
But the Tempest? The Ar 234? The B-29? the 4-hog? Those are all visibly and significantly superior unless they are being flown in a way that highlights their weakness to the maximum possible degree.
And one final issue, if we perk all the high altitude interceptors, the newer players are left with planes that are rather mediocre at altitude.