Here are a couple of links that dicuss octane rating and engine peformance, and quotes from them...
http://www.superchargersonline.com/content.asp?id=105
High octane fuel does not burn cleaner, it does not clean your engine, it does not increase horsepower or torque (unless you are experiencing knock), it does not smell better, it does not increase fuel economy (unless you are experiencing knock) and is not better for the environment. If you buy higher octane fuels for any of the above reasons, STOP!
http://www.shell.ca/code/motoring/encyclopedia/gasolines/octane.html
The value that relates most closely to actual driving conditions is the average of these two values: Road Octane Number = (RON + MON)/2. This Road Octane value is the one referred to in Shell stations: Shell Bronze gasoline has an octane rating of 87, Shell Silver is 89 and Shell Optimax Gold is 91.
Occasionally, less scrupulous Canadian gasoline outlets will use the confusion of these different octane measurements to exaggerate their octane rating claims, by advertising their fuel's Research Octane Number - which will be higher than the Road Octane Number. It is also a common practice in many European countries to advertise the Research Octane Number on their pumps, so you may see unexpectedly high octane values when travelling abroad. In Canada, motorists should always be sure that the octane number a vendor advertises is its Road Octane value, not its RON.
Engines in vehicles built for sale in North America are designed to a specified octane requirement to make sure they don't knock or ping (engine knocking reduces the amount of power it can deliver to turn the wheels). Once that octane level has been met, in normal instances your car will not experience more power or better mileage if you use a higher octane fuel.