Spitfire and Mustang questions

1. What was the difference in the wing structures of the Spitfire MkI and MkII that saw the MkIII passed over for production?

2. Any guestimates on the performance of the Mustang with one of the earlier Merlins (II, III, XX or 45 series)?

3. Any guestimates on the Spitfire’s performance if it was Mustang size and had the Mustang’s internal fuel capacity but the same Merlin for the time?