To be honest, I didn't do much comparison shopping. I stumbled upon a thread here that peaked my interest and after PM'ing back and forth with Halston of BSW I just went ahead and bought it.

My objective with this upgrade was to dramatically improve low-frequency performance while maintaining 100% storage space and stock appearance. That's exactly what I got; after wasting many gallons of fuel riding around today - I can say with absolute certainty that I'm 100% satisfied with my purchase.

I'll post up photographs of the installation later; I have company in town for Thanksgiving - I snuck away for just a moment to write this reply (wifey might neuter me if I spend any more time today in my BMW than I already have).