I made a small error last week in a filling station in the Highlands, and filled up with SuperPlus (98oct) rather than the usual standard unleaded(95oct).
I wasn't expecting any performance, I was expecting to pay more.
BUT.... riding the same types of roads, at similar speed, 2-up with full luggage boxes, the economy took a leap forward. I was averaging ~150 miles between fill ups (I fill up when the trip starts it's count down), with SuperPlus I reached 170, a 13% improvement. However, I didn't take any notice of the price.... so I can't really draw a good comparison.
I'd never normally use higher Octain fuel, like I say, this was an error on my part at the pump. I do think it's probably worth some investigation though.
(the bike has covered 1500 miles, so it's not really loosened up yet)