Why would you not consider a tune?.... 500 bucks for 20 Hp gain on a 28 is the best for your buck. I suggest going to the NA engine threads and do some reading. I personally have a 51 and before I knew that I believed I had a n52 and I was going to do a 3 stage swap which you should consider. But assuming you're looking for performance mods I recommend headers, PI and PE, and a tune.
2008 E90 N51 328i
- Depo Blackline taillights - 6000k Angel Eyes - Headlight Daytime Light Coded - Performance Exhaust - Performance Intake - AA Stage 2 -Red Cyba Scoops
- Front Windows 15% and Back Windows 3% - Two 10inch JL Audio Subs