Hi Banks,
I was hoping you would pop up.
Point taken on variable datum controller.
Rereading the Spit II manual AP1565B I still think that rated boost (highest boost without cutout pulled and not using the gate) is +9psi, and hence a 100 octane only condition.
I admit I am on less certain ground with the hypothesis that the +12psi (when approved) was not delivered for the Spit II via the cutout as per Spit I, but by making it available in normal throttle operation. AP1565B states that Spit II boost cutout is emergency use only (and does not give a boost pressure) which muddies the water a bit. I guess it could be "emergency" combat use or "emergency" boost controller failure (original purpose of the MkI cutout).
However the pilot's notes general link you posted suggest that the red tab WAS used in the Spit II for combat boost. Of course the system could just as easily be set up either way in theory.
The bit about the gate during combat is pure speculation based on the original hypothesis

If the highest useable boost is available on the normal throttle handle traverse and there is a gate, pilots would tend to go through the gate to ensure they have the maximum available, even if their height meant that the post gate throttle traverse had no effect. Subsequently they would refer to "going through the gate" to get combat boost, which does crop up in combat reports.
Cheers, camber