You definitely want to buy aftermarket, many to choose from. When you add downpipes to a n63/s63, it will get louder but also introduces an "upshift rattle". Some don't care but many do. I personally hate that raspy rattle sound it gets on upshifts when downpipes are added. It's caused by the resonators being tuned specific to the cats. You can fix by replacing the resonators and tossing in an x-pipe. Pull up a search window on this forum and search for x5m resonators. Should pull a thread from last year about all of this with videos and sound clips from another member who did downpipes.
Edit: Found the thread I was talking about :
https://www.xbimmers.com/forums/show...+rattle&page=1
I would recommend you do the downpipes first and if you hate the sound, order up the resonators and x-pipe we used In that thread. The combo of downpipes, x-pipe+ race resonators is perfect IMHO.