Originally Posted by froop
Unfortunately I don't have any direct experience with exhaust modifications as I haven't done any on my car just yet. All that I know I've gotten from research on this forum and others.
Best bet would be to check out youtube for videos of different midpipes but I'm hesitant to say how much difference it would make on your N55. Most people who change their midpipes are those with the N54 engine.
I would recommend just changing the axleback first and see whether you like it or not. That'll definitely make the most difference. If you're still looking for more then you could look at trying to change the midpipes.
I've swapped my midpipe + axleback but I did it for the sound more than any performance gains. Before this n55 135i, I had an '08 n54 135i and with that one, I never modified the midpipe or axleback, I just went with catless downpipes.
I can tell you that the downpipes make the biggest performance difference, you will actually feel the difference the first time you start the car and blip the throttle, just how much better it breathes without those catalytic converters on there. Then again, I wouldn't want to have to do that swap-work every 2 years...