wuzak
Captain
If a plane has no performance above 10,000ft then if your enemy has, he just climbs straight to 10,000ft. and looks down at you laughing. This happened to early Griffon Spitfires.
I think what Spitfire XII pilots found is that the Germans could not really distinguish between a Spitfire V, a Spitfire IX and a Spitfire XII at distance.
The 109 and 190s in service at that time could comfortable handle a V, the IX was a handful for them, particularly at higher altitudes, and the XII would span them at low altitudes.
Not being able to clearly identify which version of Spitfire was in the area, the Luftwaffe would be reluctant to engage.
PS: Not sure if Spitfire Vs remained in service in the ETO in early 1943, when the XII was deployed.