The Spit 14 will be a perk, and probably an expensive perk, unless modeled as one of the very first Spit 14's--and modeling it as such wouldn't do it justice.
With that in mind, I think I'd rather see something like a Spit 9 LF or HF as a 1944-era spit, so it could be free, or at the very least a cheap perk.
The KI-84, on the other hand, is not perk material. It essentially would be a slightly faster, but less gunned and less durable N1K2. Turning ability would be about equal to the likes of the Spit 9 and F6F (probably a hair better than the F6). Compared to the Spit 9, it'd be faster (at low altitude) and turn about as well with the same gun set, but it'd not climb as well and bleed E much faster (plus the Japanese cannons aren't quite as good as Hispanos).
KI-84 = worthy non-perk fighter that can hold its own against the AH planeset. Add it!!!
J_A_B