a Bike magazine did this test on some 160bhp litre bikes... they did it properly draining the tanks, lines, injectors etc to make sure they were using the correct stuff and not a mix... the average on the same dyno in same conditions was 3bhp increase and 1ft.lb of tourque, one bike got 4bhp extra and 1.6ft.lb which is a lot on a 170kb bike.. but in a car unless you got a 200bhp turbo car that insists in the manual u need 99RON then its a waste of money except for the cleaning the engine bit, but thats debatable..
on my 1200cc bike it does make a difference to range and power/engine smoothness which is wierd cause with the higher ron i think its faster so i hammer it more so it should drink much more, but it drinks less.. and its carbed!