I always wanted it to make a diffence too. Ya you got more perks, but it did nothing for the score.
In a slow plane you usually end up outnumbered and fighting for your life. Lets say it was a 3 on 1 and you killed all 3 while in a P40. Score-wise its less impressive than killing 5 enemy by picking,flying 5 re-arm flights of 1 kill per sortie. Probably too much to ask for the game to handle.