For the N/A, about all you can do performance-wise is :
1. CAI
2. 74mm throttle body
3. EC tune
4. SS brake lines
5. bigger brakes ( SRT6 or SLK32 AMG )
6. wider tires
7. a Sprint Booster ( makes you THINK you have extra power )
8. the new GC ( Ground Control ) handling kit.

The jury remains out on Corkie's turbo upgrade.
The exhaust ( cat-back ) upgrades will give you a meaner sound, but little if any performance improvement.