What Alan said. There are more variables in fuel than just octane. Octane doesn't make power. Octane *allows* you to do different things with the engine that make more power. By itself, octane amounts to nothing more than "unburnable stuff", which allows you to run more ignition timing and/or compression without getting into detonation. Theoretically, the less octane the better, but again, you have to look at ALL of a fuel's attributes to determine it's efficiency and power potential in a given combination.
Here's an old chart that has a lot of different fuel properties listed...
http://www.smithtex.com/racing/fuelcomp.html