The P-51D's usage seems to be hold at about the Spitfire Mk XVI's and La-7's combined usage, yet all are equally ranked at ENY 5. In Lusche's post of the time five types killed by a particular plane, the P-51D was listed six times:
Addendum: Top 5 plane kills Jan-Sep 2012
1. P-51D kills P-51D 20269
2. P-51D kills Lanc 12343
3. P-51D kills B-17 10050
4. P-51D kills Spit 16 9939
5. La-7 kills P-51D 9405
Can anything be done to encourage more diversity? Should anything be done?
I would suggest dropping the P-51D to 2 ENY or raising the ENY of every other unperked fighter by 5 and leaving the P-51D at 5 ENY by itself, but both of those have other significant, and undesirable, impacts. The first would greatly increase the number of perk points people were earning, the perk points awarded for shooting down a P-51D in a Hurricane Mk I would jump from 8 to 20, which would likely require perk costs for perked units to be raised. The second would alter the balance that ENY restrictions apply and somewhat increase the number of Fighter Perks being by low ENY fighters like La-7s and Spitfire Mk XVI's while reducing overall perks earned by high ENY fighters like the Bf109E-4 and A6M2. ENY wouldn't restrict access to things like the La-7 and Spitfire Mk XVI nearly as often.