Here is a real world comparison Spit I v Spit II both ROTOL Constant speed equipped. Hashed line the MKI Merlin III , continuous line the MKII Merlin XII. The MKXII Merlin supercharger ratio was a little different to the MKI. The MKII was also a little heavier than the MKI. The MKI values are running +6.25Lbs Boost and the MKII running +9Lbs Boost. As can be seen in both Speed and climb there is not an awful lot in it.
Sadly these values are not currently reflected in reflected in game.
Time to Height:
Speeds:
