Back a year ago and more, when super-fuels were news in Formula One auto racing, and before the present uneasy gentlemen's agreement, gains of 8-15% were spoken of, if not proven. There were certainly instances of previously uncompetitive cars suddenly becoming "runners," so it's possible the gains were substantial.
On the other hand in bike racing, the difference between fuels 'haves' and 'have-nots' are much smaller -- perhaps not even visible. Users of the $20-a-liter stuff report gains of maybe 3% and perhaps as little as 2 BHP out of a 500 cc machine's notional 170-180 BHP. Small stuff -- perhaps too small to be worth the bother; witness the 55 gallon Shell Supreme Racing drum behind the Roberts pit at the Australian GP (as opposed to all the ELF Solaize mini-drums rolling and standing all over the place, some marked as to product, others bearing only "batch numbers"). If there were a big advantage in whiz-bang fuels, everyone would have to use them.
Why the difference between situations in F1 and GP bike racing? Compression ratio may be the reason; in F1, the fuel is limited to the rough equivalent of pump gas -- 102 research octane number, with maximum oxygen content of 2% by weight, maximum nitrogen content of 1%, the rest to be entirely petroleum hydrocarbons, free of alcohols, nitro-compounds, &c. Now, 102 research ON is low performance stuff by comparison with the best antiknock fuels available; what if you start with good antiknock quality and then degrade it with something that gives a lot of energy per cubic foot of air reacted, but has itself a poor octane number? You could use a lot of it before you got all the way down to 102 RON.
On the other hand in a bike motor, regulated by the FIM or the AMA, you have no limits to octane number. That's why the very high compression ratios now in use are possible. If the energy components in a fuel tend to degrade octane number, which would you rather have; the high compression or the high energy?