I will add that the Spit IX would be better, but the 308th did have Spit VIII's in May. 31st FG would have had 2 full squads with IX's and 1 with VIII's. As far as how much service the VIII saw during Corkscrew, I do not know.
I would recommend renaming the units in the OOB. It just bothers me to see an entire FG or Gruppe be comprised of 6 pilots. I would bring it to the Squadron/Staffel level. I.e., rather than have 31st FG in Spitfire Mk. VIII's, have 31st FG/308th FS in Spit VIII's. If you change it to Mk. IX's, it would be either 307th or 309th. I will also second Devil's recommendation of placing the G-6's in JG 53 and the G-2's in JG 27. I would rename III./JG 53 to 7./JG 53 and II./JG 27 to 6./JG 27.
I also saw that bunkers and hangars are worth 3 points each. If 9 are destroyed (27 points) before Frame 3, the Allies receive .5 Victory points (please correct me if I am wrong). No problem here except that a hangar does not equal a bunker. You have hardness of bunkers at 2k lbs, where a hangar in 2,781 lbs. Considering that there are many more bunkers than there are hangars, I would either eliminate bunkers (barracks maybe?) or make them equal to hangars in terms of hardness. Thus, the hardness of bunkers would be 2.7812 in the arena settings. I say this because I would definitely tell my bombers to focus bunkers. You can effectively miss the bunkers and still achieve three points because of hardness settings versus a hangar.
Lastly, a tie is possible. If both sides win 2 frames and each side gain .5 Victory Points, it becomes a draw. It would be a draw even if one side dominated 2 frames and the other side barely won 2 frames. I would build in a VP system that disallows ties. You could also apply a tiebreaker rule.
Overall, it looks pretty good for a preliminary write up. Looking forward to it