Milling off 1mm is not a problem. Compression pressure and compression ratio are not the same thing. Compression ratio is the complete volume of the cylinder, head combustion chamber, gasket thickness ( area) etc. at bottom dead center divided by the volume it is compressed into at top dead center. This is called full swept volume and is how a 4 cycle engine is computed. The GT750 is a 2 cycle therefore actual compression doesn´t start until the exhaust port is closed thus lowering the \