MrHarris,
I think you should give the Dinan rear section a shot, since I've heard that it has the least amount of drone when you compare it to all the other systems for the e9x M3. And from what I'm understanding, the reduction/elimination of drone is your main objective when upgrading to an aftermarket exhaust.
If later you feel that you want a higher volume for the exhaust note, then I'd add the Dinan mid-pipe.......Unless you already have a different brand of mid-pipe.
I'm sure you've already done this, but I would check out H Bomb's thread once again. He has some vids of his Dinan exhaust, combined with the Dinan mid-pipe. Here it is:
http://www.m3post.com/forums/showthread.php?t=313042
I think his set-up does an awesome job of eliminating most of the drone, yet still producing a very pleasing exhaust note.
Also, I feel that getting an aftermarket
rear exhaust for the e9x M3 is mostly for getting a better sound. I don't think there's much power difference to be felt from a rear exhaust only. The main power difference will come from the tune, and the x-pipe/mid-pipe IMO.
If I were in your situation, I would not worry about how much power different rear sections provide. The bottom line is to get a better looking exhaust which also provides a better sound, yet eliminates most of the drone.
Good luck on the tough decision man.
