I don't think that the P-51 is super tough, myself. I spent some time flying it this tour, it gets toejam up about as badly as a 109 or a 190, for the most part.
I've actually been putting some thought into the Damage Modelling, because it does seem that some planes are more vulnerable to certain guns that other planes are. For instance, Spitfires seem to go down awfully easy to .50 caliber fire, but they don't show any special vulnerability to 20mm as far as I've seen (like you can hit them with a quick shot of .50 and half the time they'll go down, but they aren't any weaker than other planes are to 20mm).
I have noticed that the La5/La7 are incredibly tough, both to .50 (especially to .50) and 20mm, except for the Hispano, which knifes through everything like butter.
The P47,F4U,F6F, and F4F are also incredibly tough, but they had a reputation for being very tough. I personally think the 190 is more than a bit frail, it isn't any tougher than the 109 is, despite its repuation for being more tough.