The higher octane fuels usually have a very slightly higher calorific value and also a very slightly higher density (by virtue of the specific hydrocarbons in the blend).
In other words, since you buy fuel by the litre, you get slightly more mass for your money, and each kg contains slightly more energy.
The effect of the 2 things combined is at best 2% more energy per litre , so Grinch's numbers look in the right sort of ballpark.
However the fuel costs something like 4% more per litre (in UK), so you only get about half your money's worth back. This is in an engine without the ability to optimise the settings to take advantage of the octane difference, as the others have said.
What you do find is that some engines respond better on certain fuels, it depends on the characteristics of each engine type. I have one car which although calibrated on 91RON (and no knock system/adaptive ignition etc), it does feel much better on Optimax or V-power fuel. It doesn't produce any more power or better economy that you'd notice, it just runs better. In the other car you can't tell any difference.
I can't say I've noticed any real difference in feel in the SV when I've tried Optimax etc, but I'm not riding it every day like some folk do.
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