My opinion is that any talk of perking something like the P-51D or Spitfire Mk XVI is invalid unless it includes perking all the other late war fighters with them. Go ahead and perk those two, but perk the La-7, Yak-9U, N1K2-J, Ki-84, P-38L, P-47D-40, P-47M, P-47N, F4U-1D, F6F-5, Typhoon, Fw190D-9, Ta152H-1, Bf109G-14 and Bf109K-4 with them.
Of course, that pushes us back towards a mid-war environment, which I am all for personally. It also would mean that every country, other than Italy which was knocked out early, would have perk planes.