When I got the car it can with a Milltek back section,still having Audi cats. I have been a 3 exhaust manufacturers and all have advised I leave the cats on. Saying that because its NA its not worth doing for the price it will cost. It will sound better thats all and will loose so much torque it will feel sluggish. If it was a turbo then yes there wouldn't be a problem and I agree as my last A4 had a stainless exhaust and cat removal and it made all the difference . A Good map and decent fuel is whats recommend, I all ready had a stage 1 APR map done. So what have you other S4 owners done ? Thanks all.