I don't know if this helps, but on my 97 M3, I was told I needed a new CAT in the near future. Half of the people said universal would be fine, and the other half either said some places won't even bother installing them or they fail in 2 to 3 years. I don't know if this applies to newer BMW's, but they have tricky exhaust systems.
I know one way to tell if you have bad CATs is to either jack the car up or get under there and hit the CAT with a rubber mallet and see if you hear any rattle or noise.

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Eazy wit da Haaahdwaire!!!!