The original poster said he tested at 4000ft. The spit mkI had it's best speed at about 17 or 18000ft. That's much higher altitude.
The spitfire flys beautifully at any altitude. At least when I fly it...
maybe you just need a little practice
Someone suggested the mkI and mkIa ingame are running on 87octane at 6.25lb of boost and the mkIIa is running on 100octane at 12lbs of boost. Supposedly the mkIIa's running 12lb of boost still had an old boost gauge that only went up to 8lbs though. I vaguely remember checking this ingame.
Anyway that would still make our mkI spits more like Battle of France performance not Battle of Britain performance.
The 3d model on the mkI is a DH prop and the 3d model on the mkIa is a Rotol prop. If you look at them from outside. Same prop models as on the hurricanes.