They Daytona 955i guys fought with this for a long time. The different ways each country does the RON/MON math to come up with Octane caused some confusion years back about what the D955 actually needed.
The end result was that in the US, the right gas is 89 octane. People who routinely used 91/93 thinking they were doing their engines a favor came complaining of bad idle, less mileage, hard starting etc. No doubt this was not helped by the anemic starter fitted to the bikes. Many, including myself, noticed when changing to 89 nearly all problems resolved themselves. Anecdotal, yes, but there are statistically significant number of people who share this experience on the 2002-2006 D955i.