The octane rating of a gasoline is a measure of the fuel's resistance to auto-ignition. Auto-ignition is when the air/fuel charge in the cylinder spontaneously ignites before the sparkplug fires (called pre-ignition). This is generally the cause, with fuel injected engines, of "pinging" or "knock" in the engine. Severe pre-ignition can seriously damage the engine.
The octane requirement of an engine is mainly a factor of the compression ratio, a fixed factor of engine design which determines the compression pressure in the cylinders. The higher the ratio (or higher pressure due to super- or turbo- charging) requires higher octane rated fuel. (By the way, lower compression pressures, as when driving at high altitudes with a normally aspirated engine, can be satisfied with a lower fuel octane rating.)
So, the answer to your question is that if you are not experiencing pre-ignition ("pinging" or "knock") with 87 to 90 octane gasoline then your engine's combustion efficiency (and for that matter, power output) will not change, neither up nor down, with an higher octane fuel.
(My apologies to the major gas companies for busting your bubble.)
