My Beast is having a Milltek CAT-back exhaust and new OEM clutch installed on friday. Amazingly a BMW dealer gave the best price. Can't wait to get it back and here that exhaust. Tubi, would have been nicer but its a lot more money and there's a waiting list.
I have the Miltek's on mine, and I truly enjoy them. Not loud and obnoxious (subjective) like the Kellners, but a nice tone. You will find out very quickly if you travel in the wrong gear though The interior of the cabin has the tendancy to BOOOOOOM if you try to accelerate from too high of a gear. I think this is a symptom of all after market exhausts though.