What kind of performance are you looking for? If the stock OE muffler is "nice and open", thus not restricting any performance, and last anywhere from 10-15 years, I think that's pretty good performance.
So, what else are these stainless mufflers suppose to do? I would look to get either a BMW OE or if you want aftermarket, Eberspacher - supposedly the OEM mfr of BMW mufflers.
Btw, if you really want to spend around a grand for a muffler, don't forget to look at Dinan. His mufflers are hollowed out/gutted and supposedly are freer flowing.