A major car magazine (sorry, can't think of the name) just did a test involving the use of different grades (octane ratings) of gasoline on several contemporary engines. The engines ranged from BMW's and Saabs that have sophisticated engine management systems that automatically adjust timing to accomodate fuel grades both higher and lower than the manufacturer's recommendation, to a Dodge Ram Van with a 360 equipped with a more rudimentary engine management system. Most (but not all) engines performed better, and achieved better fuel economy, with a higher than recommended grade of gasoline. But, the improvements were just in the 2-4 percent range. A Honda engine actually demonstrated less performance and economy under these circumstances.
All engines performed more poorly with a lower than recommended octane, even those (e.g., Saab) that can be run on a lower octane according to manufacturer's recommendations.
The conclusion of the article was that using a different octane (whether higher or lower) than the manufacturer's recommendations is a waste of money, except during hot weather, a circumstance in which an engine's octane demand rises.