It will fit, it will hang lower though due to the s3 and tt having awd it needs to clear a certain part, you will also need to weld a flange to the de cat pipe for it to attach to your car back as s3 have sleeve fitment LCR's dont, the first one is the relentless one, search relentless k04 on Facebook, cheaper on there, I got one but sold it as it was a bit of hassle
 
It will fit, it will hang lower though due to the s3 and tt having awd it needs to clear a certain part, you will also need to weld a flange to the de cat pipe for it to attach to your car back as s3 have sleeve fitment LCR's dont, the first one is the relentless one, search relentless k04 on Facebook, cheaper on there, I got one but sold it as it was a bit of hassle

Could you explain the hassle please mate???
 
Getting the flange, taking it to someone to have it welded on etc, I like an easy life haha, nothing wrong with the product or service