Just to join in here, I had a Klarius rear cat(the one that sits directly under the handbrake area) fitted to mine from fleabay(£96 brand new), and nothing is showing on VCDS, I also have two front cats(that fit directly after the manifold) sitting in my garage at the moment before you go buying off overpriced suppliers on Fleabay.
My Klarius cat was delivered two days after ordering it, and to date it is absolutely fine.
I could do a front cat for £30 + postage to you, a damn sight cheaper than the one that is mentioned above!