thats an interesting question !
are you sure that there were merlins with a fixed pitch prop ? this would give the disadvantage of not reaching full rpm,s at lets say take off and with a mechanical supercharger also not full boost / power.
i believed that at least a mechanical pitch setting was available first and later replaced by a constant speed prop.
if indeed there were fixed pitch props on the first series i quess there was like written above some self regulation due to lower supercharger speed at lower rpms and so naturally lower boost.
i also quess the very first versions were not that very high boosted variants.
what i know is the fact UK was supplied quickly in the war ( i believe 1940) with high octane fuel from USA which should improve the situation.
hopefully somebody has more detailed informations !
cheers