If an engine only requires 90 octane to prevent detonation and preignition, then running 100 or 110 octane in it is not going to make it run better.
Higher octane does not equal higher horespower provided the engine's octane requirements are already being met. Having said this, 93 octane is all you'll ever need on a 9:1 engine (and you may be able to run 89 octane at the pump).
Nitromethane on the other hand is different. Becasue this fuel (which was originally used as a solvent) has oxygen bearing properties to it, the more you put in, the more power you make. This is why Top Fuel engines make the power they do, the fuel is carrying oxygen with it.
