I had after-market sports cats fitted a couple of years ago too. Audi quoted £2,500 for a pair of OE cats (not inc. fitting) when my old cats first started rattling. And by the time the rattle had gotten much worse, just a few months later, the OE cats were NLA, so I had no choice but to look at alternatives. I decided to go for a custom stainless steel exhaust in the end, complete with after-market cats. More info here:
http://forum.a8parts.co.uk/showthread.php?t=14489
I've had no problem with the exhaust passing MOTs -- I believe the important thing is that the cats should be 'E' marked/approved, which mine are. I do have to occasionally reset the EML (usually after being stuck in slow moving traffic for a long time), but I can do that by the roadside in seconds using my phone and OBDeleven device. It's a slight annoyance but a fairly rare occurrence.
It's worth noting also that you may be able to sell your old cats to a recycler, even if they're badly rattling (like mine were). I got £220 (£110 each) for mine.
EDIT: Zaustworx video of my car attached below:
And this should be a direct link to my car on Zaustworx's FB page:
https://www.facebook.com/ZaustworxCu...9764221425173/
__________________
Mark
------------------------------------------------------
2002 FE S8 Ebony Black Pearl
------------------------------------------------------
------------------------------------------------------
Cars Owned:
The Tesla Era: 2020 Model S Performance Ludicrous+ (present)
(Black, with all black premium interior and carbon fibre décor, 21" sonic carbon twin turbine wheels and FSD capability)
The Audi Era: '97 A8 4.2 (Ming Blue) --> '96 A8 4.2 QS (Dark Green) --> '02 FE S8 (present)
The Citroen Era: '84 BX 1.6 RS --> '89 BX 1.9 DTR Turbo --> '94 XM 2.0L Turbo --> '96 XM 2.0L Turbo Exclusive --> '00 Xantia Activa 2.0L Turbo
The Banger Era: '76 1.2L Lada VAZ-2101 (Ruski Fiat 124) --> '80 1.7L Morris Ital HL, finished in Ermine White and Rust