Its a bit of a topic creep (and maybe worthy of a separate thread - admins?). But this has struck a chord with me.
I knew a chap called Blunden many years ago (he played croquet with my parents). Lovely chap and highly experienced retired FAA pilot (and later, senior officer). He mentioned his Korea war service (in which I later found out he was mentioned in dispatches for his attempt to rescue a crashed pilot). As a 'plane nerd I was of course immediately transfixed and sat down with him to ask about his flying experiences. He flew a variety of aircraft during his service - including Seafire, Firefly and Sea Fury from, if memory serves, HMS Triumph.
I commented that it always seemed a bit perverse to me that the Firefly with a liquid cooled engine was tasked with ground attack, when the Fury could carry the same weight in ordinance but had a radial. His response was 'so what?'. His experience was that in actual combat conditions, a Bristol Centaurus radial was not measurably less vulnerable to a disabling hit than a liquid cooled engine. One might be a more resilient, but it also tended to make a bigger target. He said that by this stage of aircraft design, liquid cooled engines had armoured radiators and cooling lines - so the practical differences in resisting small arms fire were minimal. Being hit my anything bigger usually meant the difference was academic either way.
So in the light of that, the lack of discernible difference between P51 and F4U performing CAS seems to make sense. The idea that radial engined aircraft are inherently more resistant to damage has become something of an accepted trope and perhaps needs to be clarified.