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If you raise the compression of an engine, you will increase efficiency (all other things being equal). However, altering compression always affects other things, and you should mainly be concerned with ignition advance. If the increased compression causes pinging, the ignition timing will be retarded and you will loose any gain you might have gotten. Running higher octane fuel will help this. Its really a balancing act and until you know your engine and do testing nobody can say how far you can go. It also all depends on the environment you drive in, humidity, altitude, temperature. All those things effect ignition timing. So, what might work well one day won't the next. Thankfully, the OEMs have the resources to test for this and build in some safety factor for us.
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