You will probs see a small increase in MPG, I doubt you will see much in terms of performance, maybe a little.
Placebo effect springs to mind. I cant really notice much difference in the MK5 although I use 99 RON all the time now as I'm lead to believe its better for my engine and the FSI engines are designed to run on higher octane RON. When I changed (stopped being a tight ass) my MPG went up ever so slightly, different people seem to experience different side effects.