Could it be that the original cats fitted by the factory were pretty crap anyway?
Has anyone independently tested an original equipment AR sports cat during the three years before its MoT is due, I wonder? My sports-exhaust equiped Roadster has passed its MoTs, but 'only just' - thanks to an understanding tester.
The answer to your question is: yes, they are pretty crap!
Read
http://www.talkmorgan.com/ubbthreads.php/topics/432639/a-tale-of-3-cats