You will not get any noticeable improvement in performance using any of your domestic market fuels if your car is tuned for your home market. In the UK we have 95 RON (premium), 97 RON (super) and 98 RON (superplus) unleaded fuels available at 0.75 GB pound, 0.80 GBP and 0.89 GBP per litre respectively. There is also 98 RON lead-replacement fuel available at about 0.80 GBP per litre to replace 4-star for pre-cat cars (btw, can catalysed cars use this fuel, now that it is lead replacement, rather than leaded?). FYI 1 GBP = about US$1.5 and these prices are per litre!! The guy from Illinois should count his lucky stars!

I use 97 RON unleaded in my C200. I get better consumption than on 95 RON fuel (about 1-2mpg better), but it's no faster. My father uses 97 RON in his Porsche 996 C4 because it says so in the handbook. My mum uses 95 RON in her SLK 230 (because she won't stump up a penny she doesn't have to) with no adverse effects. I'd piss in the tank of our Range Rover if I thought it would go, that thing uses so much fuel; 95 RON will have to do.

Merc engines are built to run on grades from about 85 RON. It makes little difference what you put in them so far as performance is concerned except at the very top end. And how often do you drive to the rev limiter?