Here is a list of upgrades that I've done:
1. B&M Short shifter ($300)
2. Evo Air intake ($400)
3. Borla Exhaust ($450 for 2 day old one)
4. 2nd Cat bypass ($500)
I did it in stages over the past year, so it doesn't feel like a big hit in the pocket. If you do a search on my posts, I have video clips as well as dyno charts for the mods.
Video Clip
http://media.putfile.com/Borla-mp4 (takes a few minutes to load)
Dyno chart
Dyno results from mods
A lot of debate about my dyno charts, but it's open to intrepretations.
Other mods include a turbo look bumper.
Wish list to complete my mods in the future
1. M030 suspension ($1000)
2. powerchip ($1000)
3. R Compound tires ($?)
For everyday driving, I get the most compliments and enjoyment from my Exhaust. The Porsche shop said it sounded like a Gt3 cup car.
For Autocross and track, the suspension and the tires will be the most bang for the buck.
Hope this helps.