Higher octane retards the burn rate of fuel. This is designed to prevent preignition (knock) in high compression engines. If your engine is normally aspirated, octane above regular (87) or midgrade (89) will probably hurt performance. In unmodified turbo engines, the specified 91 (a European spec) isn't available so we use the slightly slower burning super (93). Again, anything above that will be meaningless or counterproductive. Actually, if your octane is too high I'd be concerned about burning valves, as it's possible a little combustion could still be in process when the exhaust valve opens.

If on the other hand you have a modified engine (chipped, large-bore turbo, etc.), a higher octane would be desireable. Think of the compression ratios racing fuels are designed for; unless you're approaching these kinds of numbers, you'd just be wasting your money.

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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)