Reviewing the mods in my signature, my most enjoyable mods were:
- AEM CAI - great sound
- lowering springs/strut suspension package - I'm happy with my Tokico drop (1.5"), others drop farther - improved handling tremendously
- DIY trunk brace - tightened the back end, really helped reduce understeer
- Silverstar 9005 conversion - lots more light on the road

You're already doing the catback exhaust and header; might as well drop in a high-flow cat while you're down there.