Let's make it clear, octane rating is not a measure of the quality of the fuel, it is an indication of how easily it ignites and burns. There is no particular advantage to using gat rated at a higher octane than needed, but there can be some long term disadvantages.
1. The slower ingiting properties can change the effective ignition timing. On modern bikes, you can't just loosen a bolt or 2 screws and give it a little more advance, so it could cost you a little power.
2. Slower burn can result in the fire quenching before all the fuel is fully burnt, causing carbon buildup. With heavy carbon deposits, you get hot spots that will ignite the mixture if you don't continue using high octane fuel.
3. Although it's more of a problem on the high reving smallbore multicylinder bikes, (remember that little 175 Honda that was advertized to turn something like 1700? Ever wonder why they dropped that line like a hot potato?) slower burning fuel means more heat later in the cycle, possibly enough to shorten exhaust valve life.
