As one who has worked on both US and UK Merlins the main difference from a maintaining perspective is that the US models were a little easier to work on, if memory serves me right they cut down on the number of bolt sizes and as stated on a previous post the parts were more interchangable. As for the Allison P51 "Being a pile of !!!!e", the fact is the USAAC did not foresee that fighter combat was going to take place above 10,000ft, thus the engine/airframe was optimised for this, in fact it was a very good aircraft at low level {Just check on the scores of Polish/RCAF units in this role, also its record in the low level recon role} The instalation of the Merlin with its supercharger optimised for high level did of course make it into a superlative high level escort fighter. { When instructing on Harvards I had one of the "New Luftwaffe" re- treads who had been shot down four times by Mustangs!}