Any x-pipe with the primary cats removed will help. If you're on a budget, just take a good muffler shop and have the cats removed. Probably the cheapest 20 RWHP you will pick up. The next cheapest option would be a catless x-pipe. It will be loud however. The best of all worlds is probably a titanium exaust with high flow cats like the Akra Evolution but probably going to throw you back $6K or so.
Daily: 2013 e92 M3 MW FR DCT ZCP / ESS VT2 650 S/C, MRF mid/Akra slip, ST BBK
Track/Airstrip: 2011 997.2 Turbo S PDK / 501 AWHP & 536 AWTQ Bone stock on a Mustang dyno
Beater: 2013 X5 3.5sport