The stock exhaust, outside of the EPA mandated cat's, is pretty nicely sized...there aren't too many kinks I've seen when getting down below, which is probably why most aftermarket exhausts that aren't axle-back's are netting 10 whp over the entire range...
if I were to do it, like sean10mm was saying, I'd focus on the DP - getting a 200 cell cat will give you a lot of gains WITH a strong tune.
I saw that on my Evo, turned up the boost with the stock DP, not too many gains...with a high flow cat, something like an additional 15 whp. That's CRAZY! (this is for a high boost evo, so probably not too comparable).
2012 335i Mineral Gray
M Performance Exhaust/Brakes/Suspension/LSD|Bav Stage 1/AMP||ER CP/IC/DP/OC | Dinan CAI/N55 PWG BIG TURBO|BMWF30.com