I spent almost a year reading articles, talking to friends in the car business, and asking questions of many tuners, before finding a used Velocimax 2 exhaust in my area. 

All of the information I found said that most of the "cat back" exhaust systems available are really a waste of money because they do not do any more than replace the single factory exhaust (2 1/2") with a single 1/2" (3") larger pipe...

The Velocimax is a true dual exhaust, all the back to the tips.

It sounds loud at first, but after getting used to it, it is a nice, deep, serious horsepower sound.

The increase in HP is apparent at the most important place...THE ACCELERATOR PEDAL.

I recommend this exhaust, if you can find one, and think to put other, single pipe aftermarket exhausts is a waste of your time and money....Ask the guys at your muffler shop....or read some of the technical articles found at Audi World.