a high alt flyer, given a few minutes and no other cons around the tables can be turned. if i come in co-e with a yak-9u or ki-84 i'm usually shooting him down or having to wave him goodbye.
the p51d has a good guns package and top end speed, it's only average in acceleration.
Take the fight up to 25K, up there is climbs 500ft/min better than the Yak9 and is probably 20-30 mph faster. Against the Ki-84 it's 50mph faster up there according to the charts. Granted the 51 would be at a disadvantage in a low speed turning fight on the deck but that's not the only measure of "performance".
Any 1940's area aircraft that could fly from London to Berlin fight and come home and had a top speed of 430-440mph at altitude is a high performance fighter, I think there's a hard time arguing contrary.