At the local Motor Factors t'other day I was idly clocking one of their posters on Cats/Exhaust systems whilst waiting to be served (no change there then).
Evidently using exhaust paste up-stream of the cat can lead to premature failure.
It's to do with the paste breaking into small particles and mullering the cat internals.
You live and learn!
Cheers, Pewe

well it does swell up big style like spongy foam so I can see how it would break off inside