I agree - don't change anything at this stage. Unfortunately, I don't get to look at every post, and even those I do see, it's easy to miss points which have been questioned.
The one thing which is certain, is that VHF radio was fitted to USAAF Spitfires at this period, and had been in RAF use since late August/ early September 1940, something which should show up in general research, and this is evidenced also by the lack of the wire-mounting prong on the top of the antenna mast.
As for the code letters, they would have originally been in Sky, although white was eventually adopted to come in line with standard USAAF practice, at which point the proportions of the letters was also brought in line with US standards, and these were 'heavier' than the RAF equivalent.
The Sky tail band was retained at first, but this was also removed later.
Those aircraft which transferred to the USAAF squadrons direct from the RAF 'Eagle' squadrons, would have had the roundel and fin flashed painted out, which can be seen in most contemporary photos, although your example photo lacks enough contrast to see if their is evidence of this.
However, replacement aircraft coming direct to the USAAF may not have had the RAF roundels initially, although this is unlikely, as they were painted at the factory, and then the roundel filled in with blue, retaining the yellow outer circle, and the stars added, at unit level.