Your cats are getting really hot when you smell eggs, this really means that your running a leaner exhaust mixture. It's not hurting the engine since it was designed to operate with a hotter exhaust, which might be due to the twin plugs in the head, and having a phased ignition, that allows fast flame front in the piston and more burning of fuel on the way out the door (valve) Maybe Hddp or AMG lover could elaborate in more depth since they seem to know more than I do.