You can't talk about the P-51 and just dismiss the Packard Merlin engine and the range and it's firepower... and say, "see it wasn't so great without that stuff". It did have all those things and that plus the men that flew and maintained them, made it what it was.
I never read or heard of a P-51 pilot saying they wouldn't take on a 190 at any altitude cause they were going to get waxed.