I have in the past fitted an'OE spec' cat to an Astra, 3 days before the EML came back on and the low catalyst efficiency code and description.

At about the same time we were asked to replace a manifold cat on a Fiesta, I took the old one off the car, when I compared to to the one that was to be fitted, the new one was half the diameter of the old genuine one. I point blank refused to fit it.

So Very strongly advise now - Genuine Cats, Genuine or Bosch O2 sensors