E46 performance options?

I bought my 2003 328i just last November and have been enjoying it immensely as an everyday driver. I’m a graduate student and don’t have too much money to throw at it, but I have increased the performance with a performance intake kit from Signature Motoring heat shield. This was about $170.00 and gave me a whopping 9 HP/6ft/lb increase on the dyno. The induction sound is devine on the high end but fairly normal at low to mid RPMs.

I'm thinking of a performance exhaust now from cat back. I’ve got maybe $500 to spend. Can I get something decent for this? Does anyone have tried/true recommendation? If anyone has something for sale, I'm interested. I would need something street legal and bolt on that doesn’t require any custom welding jobs. Thanks!