I've done it a week ago.
It is NOT hard to do it yourself guys! It costed me $50 in total. ( pipe and welder)
You can remove the exhaust yourself easily with a helping hand, cut open de exhaust carefully. Remove the perforated pipes with the right tools, make the new pipes in the correct size. And than find a welder who can weld everything for you! (takes the welder 1 - 2 hours)
I made a bunch of pictures of the process, if i have time i'm going to post a DIY!
I'm also working on a before/after video with revs and flyby's, but here is one video where the m3 is taking off with the modded exhaust (replaced the 4 perforated pipes)