Mod..
 

I am looking to buy a supercharge pulley for my 2005 Mini Cooper S but don't want to lose fuel mpg. Now by installing this does my MPG drop any or does it rise??

That all depends on how much your foot is on the "go" pedal. After having the reduction pulley installed, you will not want to take your foot off the gas. So, your MPG's will drop.

OK.... How much more power does it add and is it worth it or should i put a different mod on from my list? and do you have any suggestions on where to get one???

A pulley is the best bang for the buck as far as engine mods go. Your MPG wont change much if you dont hit the higher RPM's and put around. If you're a speed demon and have it to the floor everywhere, yea expect to be at the pump.

I get about 26.5 MPG with my 15% with "spirited" driving.

A pulley adds power by spinning the SC faster. The MPG you get will be in direct relation to the amount of power you pull out (aka how heavy your right foot is.)

Many vendors out there, you can buy direct from Alta, Craven, or any number of NAM vendors. If you're not going to install it yourself, you may get it from the shop who does the install, if there are any MINI shops in your area.