I'm just musing out loud, here, TA - so I'm fully prepared to be shot down in flames....
I think the problem with the Spitfire stems from the original specification ie a defensive fighter to protect the British Isles from attack.
Internal fuel capacity of the original Spitfire was 85 gallons, whereas even the early P-51s had an equivalent tankage of 150 gallons. So the Spitfire had a large deficit to make up even when drop-tanks were fitted.....the Bf109, from the same era as the Spitfire, also suffered from poor range.
I assume - although I don't know....that the P-51 was originally designed with operational use over the Pacific Ocean in mind ? In which case, it was a different horse for a different course ( excuse sickening pun
)