Here is the typical “stage” structure as followed by most tuners (fwhp=FlyWheel Horsepower):

Stage 1 (adds around 50-60 fwhp) - performance “chip”, which is merely a chip that is soldered into the car's ECU to essentially increase boost pressure and retard timing (giving the car SIGNIFICANT performance gains). To have your ECU modified in this way, you send it to a tuner (GIAC, APR, AMS/Abt, and MTM are widely regarded as the best) and they will modify it and send it back. This costs anywhere from $500-800 depending on the company. Different tuner chips give the car different power curves, so it's important to do your research before selecting which one you want. What most recommend is that you just buy a spare ECU from the tuner to have them modify. That way you have a complete unit you can simply swap out in case you need to go back to stock form. Spare ECUs run about $500 new.

Stage 2 (adds 5-15 fwhp) - just add an aftermarket exhaust to your chip (any will do). The true dual exhausts typically give the most performance gains. Cost: $600-2000 depending on manufacturer. Most do this mod simply for the sound. Here is an AW tech article on exhaust systems: http://www.audiworld.com/tech/eng28.shtml

Stage 2.5 (adds ~20-35 hp) - add replacement of stock downpipes (from turbo to cat) with aftermarket ones (with option to replace cat with high flow unit or even straight pipe). Cost: $1000-1700. Note: difficult to install

Stage 3 (brings fwhp up around 420-450) - Add turbocharger replacement, typically with K04 turbos, such as those used in the european RS4 wagon. Usually many other parts are required at this step as well; kits can be bought that contain everything (sans exhaust components) for $6000-14000.

Other common modifications include(and common brands selected): coilover suspension (H&R, Stasis), rear sway bar (Neuspeed), Snub mount (APR, what this does is to limit forward/rear transfer of weight during acceleration or gear shift), Drivetrain stabilizer or DTS (AWE, Tantrumwerks, Autospeed), performs similar function to snub mount), short shift kit (UUC, Tanoga, “Toy Guy”), air intake replacement (S-Flo), Boost Gauge (Omori matches stock lighting), and many more.

Upgrading the software on the ECU can give a significant performance increase, but also be a prime contributor to the Common Problems with this vehicle. APR Chip The setup from APR has a very distinct advantage over competing software upgrade packages being that you can change the program while the vehicle is running from inside the car. Simply hold down the 'set' button on the cruise control stalk and the check engine light will blink once from 'program one' twice for program two, etc. Each program slot can be programmed for a variety of settings including stock mode, chipped mode 91 octane, chipped mode 93 octane, race-gas mode and valet mode.

• 91 Octane Chipped - 307hp/366lb-ft (+57hp)
• 93 Octane Chipped - 318hp/382lb-ft (+68hp)

From stock to GIAC-X -

From stock to exhaust -

From stock to downpipes -