Also if ENY started at 1 or 2 the rewards for downing such an aircraft with something like a Spitfire Mk I or P-40B would be excessive. Yes, it is hard to down a Spitfire Mk XVI with a P-40B, but not that hard, not hard enough to justify 40-50 perk points for it.
I agree on the Spit 16 and such aircraft like the N1K, or all other unperked ENY 5 planes. However, I think perked planes should have a lower ENY than 5. Downing a Tempest in a P-40B should be worth way more perks than those you get for a 16.