If cost is a concern, you can just get test pipes from ACM or Turner for $400 which will extract most of the power from the exhaust. Of course this will also require modifying your stock pipe and will require welding to go back to stock. Another option is to get one of the ebay x-pipes, they may not be as high quality as the others, but fitment is usually pretty good. Here is one for $450 shipped made by topspeed:
http://www.ebay.com/itm/BMW-E92-E93-...ht_2542wt_1013
I had topspeed test pipes on my 350z and they worked great for several years. Resonators can be added on if its too raspy.
If money is no option, I see no reason not to go with the akra x pipe or a quad resonated setup like borla or dinan.