I also wouldn't agree that the P-51 arrived late in the war. Mustang Mk Is and P-51As were coming into service just after the USA entered the war. You can construct a fantasy scenario of P-51 airframes being shipped to UK and fitted with RR two stage Merlins in summer 1942. This would have reduced the number of Spitfire IXs and Mustang 1s available to the RAF and the number of P-51As and A-36s available to the USA, all four types were needed and used. Having some Merlin P-51s in mid 1942 may have been an advantage as far as a strategic bombing campaign but the USA in mid 1942 didn't have suitable bombers or the "infrastructure" to prosecute the campaign. Between the Dieppe raid in 1942 when the USA first used the B-17 in numbers and the RAF used the Allison Mustang and Spitfire IX and the start of "big week" in 1944 huge changes were made, not only to the aircraft but also to the knowledge, strategy and execution of raids. The P-51 didn't arrive late, the most photogenic of the stable, the P-51D arrived when it arrived but sometimes is given credit for what its forebears, the P-51B/C achieved in bomber escort missions. It is pilots who score victories, some flew P-47s from start to finish, some flew only P-38s some only P-51s and some flew the whole lot and others too, in Europe and elsewhere.