Just because you've had no loss of performance doesn't mean the cat isn't bad. It doesn't happen overnight, so the performance will slowly go down each day over however many years you've owned. You won't notice it as much when it gradually disappears. You've clearly got a problem that you've let linger for 9 months, so maybe what you think could simply be wrong.

I replaced the cats on my 7 w/ universal Magnaflow 2.25 in/out units w/ the morning idle, performance and exhaust note changing like night and day. The only reason why I replaced it was because I was slightly over 100k and one had started to rattle. I had "no loss in performance" either till I replaced them and felt like it was a new car again.

Another place to search is over on the 3 series forums of Bimmerforums, Bimmerfest, etc since it's the same/similar I6 motor. They've been using that motor for years before an X5 even came off the assembly line, so there is more information out there.

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