There are quite a lot of reasons to keep the stock exhaust system. It performs well, sounds pretty good for a stock system and replacing it will cost around $700 (which will usually get you 6-10 more HP too). One option is to just switch the tips out to these larger round tips to fill out the rear valance a bit. The stock tips looks a bit anemic and don't quite scream "power", in fact the way the stock tips look like the bottom of a beer can is because when one of the orginal clay models was being presented after a couple of very long days (and many chugs of beer) at BMW a model maker realized that the car did not have a muffler so in a slightly drunk fashion grabbed one of the beer cans that were sitting in the studio and cut the back off and stuck it in the clay just before a major design review. The shape stayed into production, but that does not mean you need to keep it on YOUR car. Tips come as a set of two and install in about 5 minutes with an allen key. Just reach under the tip and look for the little hole on each tip and undo the allen bolt and slide the tip off and replace with these.

Fits these MINI models:

02-06 Cooper S Hatchback
04-08 Cooper S Convertible Priced individually