I have an extra exhaust system for offroad driving with 2.5 inch downpipes, 200 cell Euro 4 cats and then a single 3 inch all the way back.

Just for fun I did MOT with that exhaust fitted and smoke level was way below the requirement for my car.
What I did notice with the standard system was that sometimes after a time of mainly stop and go traffic i could get a puff of smoke when giving it the beans on to a freeway. I guess the filters collected some soot that then was released when the pressure/exhaust flow got high. In a sense regeneration of the filter...