ghostdancer, they may seem similar, but from somebody that flew the ki61 a LOT, the Ki is somewhat inferior to the P-38 (any model). The zoom, loop, acceleration, past-stall-stability, weapons count, ammo count, overall firepower, and the overall "effectiveness" of the P-38 is well past that of the Ki.
I would limit them to 10 planes (no more). Most times we're going to see 70-120 players per side, and 26 P-38s is about 1 in every 3 or 4 of all planes, and depending on how many of those are in bombers, the ratio gets even higher for "just fighters"...
Historic, maybe, but easily unbalancing.