Quick update for the OP - I had a drive in an E92 with the M Performance Exhaust and it was just too quiet under 3,500 RPMs - very similar to stock up to that range. In doing more research, I have been told that the Akra Slip-On is louder in that range than the MPE, while the Eisenmann Sport is even louder than the ASO.
I am supposed to be meeting someone on Tues with the ASO so I will post an update after I hear it. I am VERY much looking forward to finding the right exhaust, as the car is just too quiet in the cabin with the windows and roof up! Too bad they refined the E93 M3 well beyond the old E46 M3 Convertible because now we have to add the noise back!!
I will post an update next week. Please keep us posted on what you find in your research as well - thanks.