Remember all your cats do is reduce emissions and allow you to pass the emissions test at MOT time. It does nothing for your engine except give the ECU information about exhaust gas composition.
The Lambda probe before the cat tells the ECU how well the combustion process is doing and helps adjust the mixture accordingly.
The Lambda probe after the cat passes information about how well the cat is improving emissions. If this one is defective or passes out of tolerance information then the ECU will try to compensate.
If you put after market cats on then there is a good possibility duff information will go back to the ECU because it will not get the pre-set readings it's expecting, you could well be ok but the ECU doesn't know this. To overcome this people that use after market cats modify the Lambda housings so the probes are out of the exhaust gas flow.
I'll look for the topic and post back.
__________________
Conan (the Librarian)
Watch it if you dare
Currently,
2000 S8, Merlin Purple, Leather Pack 1, Winter Pack 1, SPAX adjustable suspension, AP Racing 6 pot front caliper, 385mm fully floating disks, EBC Yellow Stuff Pads, Black Diamond grooved rear disks and predetor pads, D2 Doctors Brake Block.
1997 4.2 A8SQ - the Projekt. 12 years and ongoing!
1997 4.2 A8SQ - Ming Blue. Restoration sat waiting; saved from the Scrapyard
2003 C5 Allroad 4.2Q - Cobolt Blue. Towing car. Laid up awaiting manual conversion.
2005 C5 Allroad 2.5TDi FE - Grey - Workhorse.
2015 Q3QS SLine - Daytona Grey. Lots of extras! Long Haired Admirals cutter.
Previously
2003 C5 A6 1.9TDI FWD - Loaner from D2 Doctor Lifex
1997 S8 - Ming Blue. Loaned out and written off. I loved that car. My first engine swap after cambelt failure.
1996 4.2 A8SQ - Ming Blue. 178k on a cambelt! Trader's 8. Delivered to A8Parts at Lifex.
2003 2.8 A8SQ - powder blueish. Fill in for a while. Sold on.
1998 1.8T A4 Avant FWD. Ming Blue (there's a theme here), PEX for the 97 S8.
And a list of non VAG going back 40 years before I saw the light. 
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