The cat does nothing for smoke, sadly. Smoke particles are removed by the DPF, FAP, call it what you want. The DPF is mounted adjactent to the cat on the engine facing side.
It's purpose is to collect smoke causing particles and then burn them off periodically with the help of a fuel catalyst (Eolys, a product of Rhodia, formerly Rhone-Poulenc) and with additional fuel injection cycles during the expansion phase. It's the equivalent of a horizontal chimney fire in my opinion and is a step too far too soon.
It works, and keeps the cat soot free but adds massively to the complexity of the car.
I presume it's possible to split the particle filter and cat, replacing the DPF with a straight pipe and then re-map the ECU so that it no longer looks for excessive back pressure and does not try to fire off a regeneration cycle.
So bloody much for diesel engines being simple!!