[northernman]

I know for a fact that my KLR gets worse mileage if i have to fill up with ethanol blend gas.
I have had to a few tank fulls, and I am switching to reserve 30 to 40 kms earlier on average, than with good old gas.

And an engine that is capable of running fine on regular is best run on regular, and yes it does make more power on reg than premium, if it does not knock or ping.

The resistant to ignition of prem, also robs it of some power making potential....Would it be noticeable in normal bike riding lets say.....Doubtful, but in extreme high load cases, where you can measure it, yes it makes less.
Say strapped to a dyno, or in the case of sleds, where they cannot pul their full required rpm clutched as they were on a hard, deep snow climb.
2 strokes are more affected than 4's i notice.