VW 1.8T Exhaust systems (+10HP) - $400-800

Exhaust systems are key to a performance car.  An aftermarket exhaust is usually larger in size than the stock system.  Most exhaust systems replace everything from the catalytic converter all the way back to the tip, hence the name cat-back systems.  Replacement of the stock restrictive parts results in a free flowing exhaust and depending on the system a louder tone than stock.  There are two types of materials used in cat-back systems, the cheaper aluminum, and more expensive stainless steel.  There is no difference in preformance characteristics in a brand new system, however over time the aluminum systems will rust and corrode hurting performance, while the stainless steel systems will not rust and should last at least 10 years before replacement is required.  Stainless steel is the way to go if you want the system to last.  Expect to see around 10HP with any system.  However depending on your other mods an exhaust can help out a lot.  For example, if you have a chip, filter, and a downpipe, the addition of an exhaust will maximize all the mods you already performed. When purchasing an exhaust for a 1.8T there are either 2.5" or 3" systems available.  There is much debate as to which performs best, but it can be assumed that when used with the stock VW 1.8T ko3 turbo the 2.5" is great, but for any larger turbos you should use the 3" system for peak performance.  Note however that many people have used a 3" downpipe and cat mated to a 2.5" cat-back with excellent gains. It all depends on what your plans are and how deep your wallet is!