So what's next?
I have a few different irons in the fire, but this is what's sitting in my garage right now:
Active Autowerke headers and a Magnaflow cat-back exhaust. I picked up the Magnaflow because I've heard the BMW PE with headers is obnoxiously loud, and the Magnaflow is generally considered to have a nice deep growl somewhere between stock and the BMW PE. Should sound great with headers. The fact that it's a full cat-back is a bonus, and I had a coupon code for $200 off, which certainly didn't hurt.