I am not getting into the heglagent part as it seems to happen to properly maintained vehicles because of an engineering flaw of extreme heat build up sludging the engien ,not because it wasn;t changed on tiem which is the normal case. This is the reason they change the engine for free. So VGR you might need to redirect your ideas on this engine and its problems.
If an engine burns oil it smokes, it is second nature for a progressive burning of oil to coat the cat causing the reaction of unburnt fuel to stop activating the honeycomb as it is coated and cannot react with it to heat up .
Thus like every other motor which burns oil it kills the cat.
A motorvac might just bring the cat back if it is just dirty.