Since the P51 can easily disengage and later re-engage so he dictates the fight. It takes a bit of trickery to make a P51 bleed enough E where he can't extend away, outside guns range, and most don't fall for it. As the old saying goes, speed dictates the fight if the P51 wants to stay out of harms way, with half a brain he will be able to do so.
"Most don't fall for it" means most P-51s one pass haul bellybutton at 500 mph and are never a credible threat. If the Pony actually engages in such a manner he has a shot, the P-38 has quite a good chance of turning the tables.
People argue that speed is everything in this game, but the Pony is not faster than a D9, K4, La7, or Typhoon, and the
2nd most popular plane is the only moderately fast SpitXVI