Well, finally got an S4 4.2 V8 and wow what a machine, never been so proud of a car. after all the research I've decided to have a milltek non resonated cat back in two weeks. I was worried about the droning however I have the tiptronic gearbox and can't find any reviews on this. I will update my opinion when it's installed and tested. if i don't like it, it's going to cost around £130 to have the resonator installed.