honestly speaking, the P-51D is a lot easier to deal with then the Spit16. But both can be beat with patience and good flying. Planes like th tempest however, are just too powerful to not perk - and thats the difference.
I hate the perk system, it encouages grinding and vulching and other relativly poor behavior - I don't mind vulching per se because its a great way to shut down an airfield - but - if things got perked, then it would encourage the behavior - and that would kill the game.
So I say, quit running to the Perk it button everytime you die to a plane. The 51 and the spit are not unbeatable, learn your plane, and dictate your fight - and if you die - just up again and try again. . . you can only get better if you learn from you mistakes.
I also love seeing 51s, they are fun to fight, and I don not mind people hitting the deck and running - since I am usually headed for the base they are running towards anyway - so sooner or later they will have to fight me. . .