The P-51's reputation is NOT propaganda. When it entered service it was much faster than its typical opposition while exhibiting climb and turn parity with them (at altitude.). IOW, it was every fighter pilot's wet dream.
The Pony was typically a generation ahead of other fighters of the era and lasted longer I beleieve than any other in front line service with admitedly second rate military powers.
It has speed, range, manovreability but above all it had numbers. There is no comparable air craft on the allied or axis side.
One of its great strenghts in game is that is discourages you from getting into energy sapping dog fights which increases your chances of RTB. US fighter pilots would be looking to escourt bombers and kill german fighters ideally on the ground though the Pony used correclty is a match for all but the 262 in the air. If they outnumbered the enemy 2 or 3 to 1 I am sure they didn't mind