Eivind Soltvedt:
High octane gasoline does not produce more HP than regular grade or any other gasoline. If an engine runs without pre-ignition/detonation with say 91 no. octane, using 98 or 100 octane does nothing but waste your hard earned money.
This article might enlighten you on the subject.
http://www.buy-sell-repair-cars.com/octane.html
JackD