The exhaust will not shoot out any flames. Where do the stock headers dump out? If MB's have shorty headers, it might not be such a good idea.
My friend tried it for 10 mins on a C320 and it sounded like a monster, but it was in no way for an extended time.
It's probably a better idea for you to get cutouts. If you don't want to spend the $400 for the electrics, get the manual set from Summit Racing for $70 a pair. $50 to install and for $120 you can have your fun regularly. It's just three screws and a reusable gasket so you can open/close them anytime, and not be scared of exhaust leaks either.
For the ultimate peace of mind, just pop them in after the cats. I know that will quiet it down some, but you won't have any issues with the rear O2 sensors. I don't know about the MB, but on my Buick with that same setup, I couldn't hear people shouting at me at stoplights at idle.
It'll be the most fun you'll ever have with that car, for sure :thumbup: