The Mustang only worked because the British replaced the Allison engine with a Merlin as an experiment
Based on the RAF experience using the Allison powered version for Rhubarb raids there was a USAAF suggestion put forward to retain an Allison powered version in production along side the Merlin version for the low altitude/ground attack role. Reason being better fuel economy with the Allison (longer range) as the Merlin couldn't run at the low RPM necessary. the Allison was more bullet proof, averaging 1,500 hours between main bearing failures vice 5-600 on the Merlin. The RAF removed the automatic boost controller so they could extend the 44" limit out to an approved war emergency 56". On operations the RAF operated for up to 20 minutes full throttle at sea level (72") without hurting the engine. The aircraft was faster than the 109 and 190 of the time (May 1943) and the rule was not to enter combat, but outpace the enemy by "running away" as it were. The ground attack objective being seen as more important. Even though the RAF Allison version remained in service right up to VE Day and was involved in D Day and subsequent operations, continued production ceased with the Merlin introduction. Merlins used for the Reno races use Allison con rods because of their better ability to handle the stress.
The cooler drag in particular is interesting, even with the 'meredith' radiators, the difference is that high?
The difference is down to the implementation of the Meredith design. The Spitfire intake was swallowing the boundary layer, whereas the F-51 inlet was positioned out of the boundary layer, and because of the larger radiator in the -51 the air passing through had a greater dwell time so the air temperature was able to be raised to higher temperature. The reason why there was a proposition to give the Spitfire the -51 style set up.