Indeed, the Mk.III was 'right on the money', showing how the small details can make a lot.
It did featured fully covered U/C (similar to the prototype on main wheels), and the bullet proof windshield was internal type, rather than external that again cost some speed. The wing tips were clipped on the Mk III so the wing was with a smaller span area, probably earned there a few mph. Being the prototype, it's fit finish was probably excellent
Too bad it was not further pursued with Merlin 45 (so it doesn't not draw the Merlin 20s needed for bombers), with pressure inject carb (no problem with negative G, less losses than with float type) and 'single' exhaust stacks, reliably beating 400 mph mark even when in mass production.
Overall agree, though the 'standard wing' ended up being preferred, with wings clipped as required. Another improvement was a better radiator, long a major drag issue (compared to a Mustang or a Mosquito). The Mk III would have been a very good match for the 190A's of the time.
Supermarine did propose (in late '43 from memory) to change the Spit to have a Mustang like radiator system and estimated it to add 25+mph, but MAP canned it due to production desruption and RR just kept coming up with more power.
The other major innovation was more range, this meant jamming more fuel into it and many good prototypes were developed. But once Portal was in charge of the RAF no LR Spit (except PR ones) were going to be in production, as he hang his hat (to Churchill, Arnold, etc) that a LR figher 'was impossible'. When the boss says no and his prestige is on the line, it is not going to happen. The Germans were just as stupid it has to be added.
Do you know the idiot was still arguing (in writng to Arnold) that it was 'impossible' as late as Sept '43, just as the P-51Bs were coming into play?
Very late model Spits (XVI for example) did finally have rear fuel tanks but with ridiculous operating limitations.
But a Mk V with a 250-300 mile combat range and a Mk VIII/IX with 500 miles was technically possible (and prototyped). Once they got a handle on the CoG issues (using bob weights and a revised elevator horn), the CoG problem only lasted until enough fuel (about 75%) in a rear tank was burned off in climb and iniital cruise. Exactly the same as a Mustang which was nearly as horrible until it did the prety much the same.
Because of its large wing tanks and lower drag a Mustang would always have a better range but you could trick up a Merlin 60 series Spit to about 70%-75% of that. Still very useful in (say) early/mid '43.
A longer range Mk V would have enabled daylight (and hence more accurate) raids on the French U Boat pens before they were hardened, arguably the single greatest strategic mistake of the whole British air war effort, heck a huge mistake full stop.