Well Pat,
You are correct that, near the end of the war, they figured out how to get at least SOME decent range from Spitfires. Since they did that, I'm pretty sure you are correct that they COULD have done it earlier than they did in real life. The U.K. was never never really flush with Spitfires and pilots at the same time and, had they found this range earlier, then yes, some Spitfires COULD have been re-tasked with different missions.
I am just pointing out that, had they done so, you would have to factor in the Spitfire missions that didn't get flown due to the retasking. It makes for an interesting "what if," except for the fact that they HAD longer-range fighters that DID fly those missions, and we'd have to speculate whether or not the Spitfires would add some dimension that was lacking in the missions flown by other fighters.
Suppose some missions flown by Mustangs or Lightnings, et al, were to be retasked with Spitfires. Would they get better results than were achieved or did the Mustangs, Lightnings ,et al, accomplish the mission? If they DID accomplish the mission, what would the Spitfires add that was missing in the original mission? If they didn't accomplish the mission, why do we think Spitfires would have done so when the mission failed in the event when flown by other aircraft?
I'm trying to understand what adding range to the Spitfire earlier than they did in real life would change, and I'm having a hard time flushing out the benefit that would accrue over and above what was actually accomplished by other aircraft. If you replace 300 P-51s with 300 Spitfires on an escort mission, would they do it better? By the time we were flying 300-plane escort missions, the Luftwaffe wasn't exactly in a position of air superiority, and the losses were pretty low. Some of the losses we HAD were due to Me 262 kills, and I'm not really sure Spitfires could have done any better than P-51s did against Me 262s. Maybe, maybe not.
I AM sure the Spitfire pilots would all have felt better with some extra fuel aboard and I AM sure they would have found a way to take advantage of the extra range afforded by the extra fuel. So, your point is taken that, yes, more fuel in Spitfires earlier than happened in real life would have been better for the Allies.
We could have had a huge impact had we started flying Douglas Skyraiders in 1943 escorted by your longer-range Spitfires. But, that's another thread ...