Hi,
Before Rover went tits-up, all the 1275 head-gaskets were upgraded to turbo spec, so one head gasket did everything.
The compression is changed by the size of the dish in the piston, as all heads left the factory with around 24.4cc's, turbo or normally asperated. the turbo head differs from that of a normally asperated head by having thinner port/combustion chamber walls to allow more heat to be transfered to the coolant.