Having 20 more gallons of fuel in an under seat/rear tank allows Spitfires to be climbing before the Luftwaffe crosses the channel in 1940 instead of waiting until the last minute and climb up from underneath, in 1941 extra fuel allows MkV's to maintain higher speeds over France to negate to a degree being bounced so easily, in 1942-43 MkIX's could have been doing deep raids into France supporting the daylight bombers, more fuel allows more flexibility, it's that simple.
I think you are trying to solve a problem the RAF did not have in 1940. The limiting factor for RAF fighter responses during the Battle of Britain was not range, but how good they thought they knew what the Luftwaffe was going do to over the next hour or two. Given the ranges involved the Luftwaffe bombers could stay in a holding pattern over France, fake setting course for England before returning to the holding pattern and still have the fuel to conduct a raid. The Luftwaffe could run feints, try saturation of an area, timing multiple raids to catch RAF aircraft on the ground, make course changes and so on, with a limited fighter defence it was always a judgement call about when to put fighters into the air, too early and they could be reacting to a decoy or be left behind by a course change. A main point of the system was to minimise flying hours and so strain on pilots and ground crew.
On an individual level having more fuel is a good thing. For the RAF as an Air Force in 1941 and into 1942 the Spitfire range was not much of a missed opportunity, it was in the second half of 1941 for the draw down of Luftwaffe day defences in France, the Germans had little in France etc. they were worried about and if they did the RAF lacked the bomber force to do much to them, even counting all of Bomber Command. Malta had the enemy coming to them, overseas Spitfire fighters elsewhere were a second half of 1942 event and most such day operations were happening within Spitfire V range. Getting working drop tanks was enough at the air force level of things but agreed more internal fuel was more efficient, though the mark V was not matching the performance of the German fighters.
The mark VIII helped solve the range issue and it arrived at the end of 1942, plus there was a build up of allied day bomber strength. My opinion of the optimal 1943 and beyond Spitfire change schedule, given what the RAF did, would be make wing tank kits available for earlier Spitfires, switch Castle Bromwich to the mark VIII, not IX (or at least the IX being built has the VIII internal fuel), by or in Q4 of 1943 enable the mark VIII to fight with a full 33 gallon rear fuselage tank, this would enable the Spitfire to keep matching the P-47 range for operations from England into 1944, in early 1944 enable a 1,000 pound bomb load, ideally with the rear fuselage tank full, to allow longer range fighter bomber operations, again mostly from England, then in the second half of 1944 put in the larger rear fuselage tanks to enable Spitfire escort from England of Bomber Command raids on the Ruhr. That matches the historical operations available for the Spitfires to do in quantity, plus the opportunity to enable longer range RAF day bomber raids in 1943 and fill the gaps escorting 8th Air Force bombers in the second half of 1943, but also note the limits at the time on the bomber strength in terms of size and weather as well as doctrine. In the period January to May 1943 while still under Bomber Command the RAF day bomber force dropped 1,230 long tons of bombs, the US medium and light bomber operations essentially started in July 1943, managed 8,179 short tons of bombs for the year, all up the 8th and 9th Air Force bombers dropped 22,440 tons on France and 767 tons on the low countries in 1943. There were still few targets in France, Belgium and the Netherlands the Germans needed to defend though, defence was more about attritioning the allied bomber force. The US heavies dropped 47,452 short tons of bombs in 1943, over half in the final quarter of the year. Not a lot of point having long range escort fighters and few bombers available.
Look at the RAF Merlin P-51 operations in late 1943 and early 1944 when considering what the longer range Spitfire would have done out of England, the RAF P-51 units certainly did operations in support of the 8th Air Force raids, but as far as I know that was only a part of their work, Operation Overlord was more the priority. The same would apply to any long range Spitfires in the same time period. In any case these P-51 were a better choice to replace long range Spitfires in the escort role as soon as the P-51 were operational.