Not much of world beater. The Mustang was about a ton heavier than a 1941 109. Powered by the same engine it would have been in trouble. Good aerodynamics only gets you so far. A power to weight ratio 33% worse than your opponent is a big handicap.
How is that? The P-51A had 1,150hp Allison and made 390mph, IIRC at 18,000ft. With a DB601 she would still not have the vertical mobility of a 109 but she´d be the best US-made fighter by a large margin. And if the Me109F can make 418mph@21,000ft, the heavier but laminar flow wing Mustang would hardly be slower.