Megan, it would be interesting to see how a P-51 with a minimum drag in the shape of a streamlined nose w/o any cooling configuration, as a benchmark, would compare to the P-51 with the existing configuration and a P-51 with a fictitious radial engine with aircooled cilinders. I wouldn't be surprised if the Merlin version even with the large cooler-cum-thrust would be better than the radial engine configuration. The question renmains why the P-51 was so much more favourable drag-wise , than the Spitfire.