The reason you would lose HP if running too much octane on a low compression engine is because you will pushing un-burnt fuel out the exhaust before it has time to completely burn in the combustion chamber.....
everything from cam timing, to ignition timing and curve, to pipe flow and backpressure ALL have to do with this...
cramming as MUCH air/fuel into the combustion chamber as possible.....then, igniting that A/F mix at the precise moment in order for it to begin its burn rate and push the piston downward in the cylinder ....then, as soon as it is completed, leave the combustion chamber out the tail pipe so the next fresh A/F charge can enter......and it starts all over again...
with this in mind....if you are using TOO high of octane...which as we all know, the higher the octane, the slower the burn rate...(kinda like magnum gunpowder)....this fuel cannot COMPLETELY ignite and be consumed at the same rate as lower octane fuel.....so some of this fuel is going to be wasted.....or the timing of the burn rate is going to be off.... as you increase compression....then this fuel can burn at a faster rate due to increased pressure inside the cylinder......where a low octane/faster burning fuel, would cause detonation/preignition from burning TOO fast....
High octane fuel is great...but only if needed.....
does any of that make any sense?
Greg