Yes, all true that. Certainly fitting 79 mm pistons will return far more HP than shaving the head and double-valve springs -- or even changing the camshaft.

Note that H75 and H89 can both have the same compression ratio depending on the year/region of the H75 (there are many H75 heads). For best power and fuel economy go with a closed chamber head. Control your compression ratio with the piston dish -- using flat-tops to raise compression often results in better power than milling the head.