I've owned 5 Mercedes from an '87 300E to my current '97 CL600. I have NEVER had a problem with passing "smog". And I've never done anything out of the ordinary except to drive it to the smog station in a lower gear to get the cat HOT!
In fact, I just bought this '97 and was watching the test dweeb as he ran the tests. I noticed he walked over and tapped (I assume) a gauge. The car passed with flying colors (12 cylinder 120). I looked at the results. Most of them read "0". I can see why he tapped the gauge since the readings were almost perfect.
Anziani
'97 CL600 50K