Well after some more testing of the Mk IIa I must say it is modelled perfectly on a real +12 lbs boost Spitfire Mk IIa for top speeds.
These numbers are taken at full CEM and engine overheat enabled. *Warning* Disabling CEM or overheat results in a Spitfire Mk IIa capable of 390MPH. Do not use CEM or overheat OFF for speed testing.
As can be seen the Mk IIa speeds are spot on when compared to a +12 lbs boost Spitfire Mk Ia. The speeds are around 20-25mph faster than the 9 Lbs boost Mk II a speeds which would put them about right for 12 lbs boost. Also at the full throttle height of around 18,000 ft the boost there should be no speed difference between 9 lbs or 12 lbs boost.
The Mk IIa in CoD (at least in speed for altitude) is definately a 12 lbs boost version. The guage in CoD is broken. I will add climb tests later.