Blue Mako,
I agree, and in my opinion the P-51B should have a higher ENY value, say 30 or maybe even 32 or 33. I certainly don't think that an aircraft with the performance of the P-51B should have an ENY of 45.
ENY ratings must be a subjective combination of performance and usage. Nothing else is fair. They need to reflect both how common the aircraft is in the AH MA and how easily that aircraft can be successful against the other aircraft in the AH MA. Only when both are considered are the perk vales able to be set fairly.
How different, how much less difficult, is it to kill a P-51B than a P-51D while flying a C.202 or A6M5b? Not very, and the effort and skill in having done so needs to be reflected in the perk reward.