I've built a small handful of engines at around 10:1 that ran fine on 87 octane; Saturn 8-valve, Saturn 16-valve, Chrysler 2.2 TBI, a Honda 2.0 Turbo (yes, around 10:1 w/turbo), Chrysler 360, and am finishing up a Jeep 4.6 stroker (based on the 4.0) @ 10.2:1. My tricks lie in the cylinder head work, with special consideration given to the combustion chambers.
The George Arlington Moore trick of extending the intake lobe part way into the compression stroke to bleed off some of the pressure may be feasible with a custom ground cam. At any rate, the higher the CR, the higher the combustion efficiency -- within the limits of the fuel AND engine design.
Jeep combustion chamber:
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Chrysler 2.2:
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Saturn 8-Valve:
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