Not at all against the grain. Did you conduct a direct measure of the combustion chamber and account for the volumes of the valve pockets in the piston crown as well as the actual piston deck height and between the cylinder and piston crown above the rings? They all need to be accounted for to get an accurate value. A typical way to do this is to get the engine at TDC with the valves closed and then use graduated syringe, graduated pipette or burette with pinch clamp (best) and direct fill the combustion chamber with an oil such as ATF. With the Commando, the plug hole needs to be facing up to avoid trapped air. 850 Commandos came from the factory with a flame ring head gasket, correct? As stated above, I was only going with anecdotal evidence from this forum - maybe a risky thing, eh? So bumping the compression above 8.5:1 really would be a modification from stock. A good valve seat job could also be construed as modification from stock but both "improvements" alone, and certainly together, will render improvements in power. Whether they make the difference between 108 mph and 118 mph I do not know for fact.