I went with a straight-through (magna-flow I think) from my local muffler shop, cost me under $300 and sounds every bit as good as the PE IMO. I've heard one or 2 other 135's and 335's with exhausts and mine sounded as good or better.
The muffler shop held 4 or 5 different mufflers up to the pipe so I could hear how each one sounded. The car isn't that loud even with no muffler actually. I went with the straight-through tube style muffler.
The sound is very nice, drone isn't bad at all, and it has a nice burble off-power.
Axle back exhausts are basically a tube and a muffler, I just couldn't see paying that much.
The BMW exhaust weighs a LOT, just something else to keep in mind.