Hiya
Mine developed a pretty mild rattle at idle - mainly when hot - and performance wasn't altered. I think usually that only happens if the cat breaks up and a piece gets stuck blocking the exhaust.
My car had a Milltek and I did notice it was a little more metallic sounding with the new metal cats (sounded better to me!) but I didn't do that many miles on them before I sold the car.
Hopefully the cats are working well - current owner may chip in 
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===SOLD===2002 Audi S8 D2 Final Edition (yes, really) in Avus Silver with purple extended leather===SOLD===
2011 S5 Sportback in Phantom Black with black Super Sports leather, 9x20s, tech pack high, adaptive xenon plus, intelligent key, memory seats pack, sunroof, B&O, Audi Drive Select & quattro Sports Diff, DAB, parking system plus
2015 VW Golf GTI Performance Pack in Carbon Grey with black Vienna leather, tech pack (Discovery Pro nav & Dynaudio), DCC, factory towbar and retrofit RVC
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