This is true for Rank III.
But this is a conceptual & design disadvantage which was also an irl issue. But in deviation to irl - in wt almost nobody is able or willing to invest time in learning tactics how to play vs slower, but better climbing and better turning aircraft.
It is also true that the US air tree has the most fighters with interceptor & air superiority spawn (similar, but different) in the WW 2 range in order to counter this disadvantage - together with artificially lowered BRs (in Air RB) of non-air spawn fighters due to the high number of rookies.
I see the US Spits as historically justified, but also kind of detrimental for development of pilot skills of fresh pilots as planes like Spits or captured A6M2s / Ki-61s/ Ki-43s allow them to skip the learning experience of USN pilots in the Pacific or USAAF pilots in Europe - they managed to reduce their combat losses by using teamwork an tactics.
Nevertheless: +1
3 things on top:
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As there are 109s, P-47s and P-51s in all hell of different nations - a second Spit would not kill the immersion - outside the known issues for SB pilots (still a niche mode) immersion is already dead. I mean if i see US players in premium 109s on Iwo Jiima or Saipan (or land based short range UK Spits and Typhoons) you might confirm that there is no immersion.
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I do agree that the 3.3 (Air RB) F6F-5 is an outstanding turnfighter - as soon as the UK premium version returns to the War Bond shop i will buy it simply because the premium version is Rank III.
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If your goal is to improve the US prop TT - adding new aircraft is imho not really the right path - a major improvement would be to upgrade some aircraft to Rank III. A lot of actually quite good and capable aircraft were reduced to Rank II in the recent years whilst other nations have them as Rank III premiums. Imho gaijin is just milking US rookies with this strategy.