I would recommend first evaluating what you want to improve on the car and how much you want to invest. Do you want to improve stoplight to stoplight? Have more fun on the on-off ramps and corners? Is streetablility and comfort any concerns? Answers to questions like those will help focus where you spend your dollars. For some ideas on the 05+ cars, assuming this is a primarily street driven car, and your looking for low-moderatly priced upgrades, I would recommend the below items. You should notice a 'seat of the pants' difference.
- short throw shifter (if manual)
- CAI and Tune. Avoid any CAI that does not require a tune. Use a good tuner. The tune will also improve items like throttle response, shift points (if auto), rev-limit, speedometer correction for gears, etc.
- Caster Camber Plates
- Adjust. Panhard Bar
- Adjust. Shocks/struts
- GT Rotors/bracket (upgrades to GT brakes)
- SS Lines
- Hawk HPS brake pads
For exhaust, the only real restriction point is the axle-back component. You can cheaply upgrade to a GT axle-back for a minimal gain. Anything after that is for looks and sounds (dual exhaust will not gain you anymore HP). Avoid any CAI that does not require a tune as you will see very small (if any) gains.
Of course you always have the higher performance options available (wilwood 6 piston brakes, Supercharger, etc) and that will drive your invest $$ up as well.