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Compression ratio is fixed by crankshaft stroke, deck height, and combustion chamber dimensions. The DME alters ignition timing based on pre-ignition as detected by knock sensors; pre-ignition, or knocking, is impacted by the octane rating of the fuel.
Octane is frequently misunderstood. It is only a measure of the resistance of the fuel to pre-ignition, i.e., higher octane fuels are more resistant to ignition. This allows higher compression and cylinder temps to be utilized to increase specific output. All things being equal, octane by itself has no impact on power.
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