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.