Only planes I have seen losing parts more tougher than P-51, is N1K2 and F4u's.
Spits loses horizontal stab with one 20mm hit into either of horizontal stabs.
(talk about weak butt)
N1K2s takes alot punishment also, specially if its .50 caliber, never seen any fighter in AH take as much .50s as N1K2 does.
F4U's, I've been chopping them part by part couple of times

P-51, definetly tougher than 109 or spitfire, about equal with 190, though, 190 is smaller target and less visible than P-51 (maybe its just the colour of P-51, not that 190 is tougher?

)
Lets also remember that you DO NOT hear exact number of hits.
This goes up to lag & code.
So its kind of crap to say "oh toejam, he got me with 2 pings!" (which can be like 15 hits)
If you wan't quick kills with poor gunnery, turn into cannon hog, it has ammo and guns.
I Have had no problems downing fighters with machineguns, not even with 7.92mm or 13mm guns, which both are weaker than brownings .50 cal.