Without recieving some major boosts to internal fuel, the Spitfire couldn't of made a long range escort like the P-51/P-38 and to a lesser extent the P-47.
But, by the middle of 1944, with the fitting of wing tanks, rear fuselage tanks and enlarged foward tanks, Spitfires had double their internal fuel capacity, and with the fitting of external tanks its range had increased by around 250%. Spitfires VIIs were running 4 hour long daylight escort missions by mid 1943.
Did it turn into a long range escort fighter? No, of course not. But its not as useless in the post '43 ETO/PTO/MTO as syscom would have us believe.
Similarly, while the Spitfire couldn't operate effectively over Germany as a long range escort in 1944, the P-51 could hardly of fulfilled the role of point interceptor that the Spitfire played over Britain in 1940, Malta in 1942 and over the Continent in the post D-Day period.
Its really a case of design priorities. Spitfires were designed as short ranged point interceptors. The P-51 was designed for long range operations.
The Spitfire pushed the LuftWaffe back into France and then kept it there. Can you imagine the USAAF trying to to build up its first mass bomber formations in late 1942 and 1943 without the RAF running constant interference? Or getting masses groups of P-51s, P-47s and P-38s into the air without the RAF controling British airspace?
The excellence of the Spitfire and the dedication of the RAF allowed the USAAF to concentrate on destroying the LUftWaffe. Someone else was running defence, so they could concentrate on offence. If the USAAF were forced to defend their own airspace, do you think they could of projected power nearly as effectively as they did?