I have been reading up on exhaust systems for my S3 and need some advice. I have a full stainless exhaust from the turbo back that was on my car when I bought it. On closer inspection it would seem that this does not have a catalytic convertor on it and I am need one for it to pass an MOT in September. I have spoken with a local exhaust shop and they can weld in either a 400, 200 or 100 cell catalytic convertor, but I am unsure which one to go for. would the 200 cell be a good compromise? Or should I play it safe and go for the normal 400 cell one?