Here's someone's post I saved a while back about fixing cats on a 300E. I don't know if it's applicable....
"Your catalytic converter(s) are clogged- probably the small ones in the
front pipe. This happens in a 300E from oil consumption over a long period
of time. A quick cure (other than the obvious one of buying a new front
pipe for $1,200 or so) is to take the pipe off the car, and have a shop
glass-bead the front (engine) side of the mini-catalysts a couple of
millimeters. The clogging seldom goes deeper. Proper performance is
restored without damaging the catalytic effects."