I've been in 1Ms with catless downpipes -- once in combination with a N55 midpipe and another with the stock midpipe.
In both situations, the sound was fine. A little deeper than stock with bit of "gurgling" and significantly less drone.
The smell of gas was, of course, apparent with the fully catless set-up. However, with the stock (catted) midpipe, it was only detectable upon start-up.