Higher octane denotes a fuel's ability to resist knock or preignition at higher peak combustion temp and pressure - pure and simple. It has little to do with burn rate or flame front velocity.

Higher octane fuels allow the use of higher Compression Ratio precisely because of it's ability to withstand higher peak cyl temp and pressure before knocking. The power gain comes from the higher thermodynamic efficiency derived more complete expansion of the combustion gases - inherent to high CR. Additionally, higher octane may allow the ignition timing to be advanced further, thus creating even higher peak cyl temp & pressure. Most modern EFI engine have knocking sensor(s) that allows the engine to run at the hairy edge of impending knock - maximizing power and efficiency.

With a lower CR engine (like our weedies), you can advance the timing all you want and never get close to the peak combustion temp and pressure rating of the high octane fuel. Beyond a certain point, advancing the ignition timing further will only do more negative work, which manifest itself in higher engine running temp. As long as you don't over advance the timing, running higher octane fuel in an engine which cannot take advantage of it will do it absolutely no harm (the lead content is another story). It will be like flying a Mach 3 capable fighter jet only at Mach 2. Nothing wrong with it but it's kind of a waste.

All this is, of course, all theoretical. In reality, much of it depends upon a myriad of operating conditions - atmospheric, intake and exhaust configurations, cooling loads, and even carbon deposits can affect the onset of knock and alter an engine's octane requirement. Feel free to experiment. You might just find a particular engine setup on a particularly hot and humid summer day doing much better with heavy avy fuel.