[Muskys SS]

A lot of really good posts on this issue!
On a side note, 91 octane gas from the US has more detonation resistance then 94 octane from Canada. A lot of 91 octane tunes from cobb,diablosport,etc... will detonate on our 94 octane. This is a heads up for you turbocharged guys uploading pre-programmed tunes. Fortunately a lot of the newer vehicles have good knock detection and can retard timing to save your engine. But remember no 2 cars are identical. Always check your new tunes on a dyno where gains/losses and detonation can be monitored.

Back to your original question Mugen Evolution. Most late model naturally aspirated honda/acura engines can reach mean best timing a few degrees before detonation occurs. That means chances are you won't benefit from a higher octane fuel assuming your tune is correct.