A X pipe isn't a bad idea, but don't expect additional performance from it. And sonny is talking out his sphincter. A stage 2 S4 is going to run mid-11s at near 120mph in the 1/4 mile, and 0-60mph in a hair under 4 seconds. It has a 40:60 torque split drivetrain that can vary up to 20% either way and a torque-vectoring differential. At that level, it's not so far off (but not fully equivalent) to the everyday performance of a gen-1 GT-R. It won't work on a race track as well without some work, but it's a lot nicer car to drive, day to day. Not that it's really a comparison I'd make, other than to say that you can accelerate with that kind of urgency, which was my point.
It is definitely *not* a tarted up A4. The entire drivetrain is different, the brakes are different, wheels are different, the seats are different, most suspension pieces are different. The front clip is different, there's a rear air diffuser... A tarted-up A4 is the S-Line model.
Here's a guy who claims to have a "world record" with his S4. Did this shortly after GIAC released a new tune revision.
[video=youtube;EBVwHWooa3Q]https://www.youtube.com/watch?v=EBVwHWooa3Q[/video]
These cars do not respond to exhaust mods, and his intake is the same I've bought, which does have a small effect on power (10hp, perhaps but more consistency which makes it worthwhile). In other words, it's not a remarkable car. Next year with the new Stage 2 tune available and the 7-speed DSG tune, I expect them to be in the low 11.4s with the DSG version of the car and perhaps an intake. Set the world on fire? No. Make for a hell of a quick daily driver? Oh yes. And with the quattro drivetrain, you can use all of it, pretty much any time you want.
BTW, if you go searching for videos or data on the tuned cars, disregard anything before about October last year. The tune companies have refined and improved their tunes for these cars a good deal in the past few months. I'd rate the GIAC, Revo and Unitronic tunes as being the strongest right now, with the nod going to GIAC for overall performance, and Revo for overall features. Unitronic and EPl are both "young" tunes in their early revisions, but EPl's tune is flash-at-home so you can flash back to stock for a dealership visit if you have a warranty... Unitronic's will soon follow, and the current state of the tune itself is very good.
Or leave it untuned if you wish, but you are leaving a lot of performance and excitement on the ground with this car.