The P38 has limitations.  The P51 is actually a better plane.  The max dive speed of the P51 gives it a big advantage over the 38.  Its fuel economy, top speed, and pilot visibility give are also better than the P38s.  Now I have no hard proof, but I believe that that with a P51, you could consistently bring home more kills per sortie than a P38 and not get killed.  Back during the AH beta, there was a guy who averaged 10 kills per sortie in the P51, and thats no lie either.