My "new" 190E 2.3 1992 belonged to my brother-in-law, who assures me that that car was once adjusted so the idle was at 1000 rpm, and ran noticeable better, at the expense of a higher fuel consumption. But it had to be readjusted again so it could pass the emissions test.
The only thing that I can imagine the garage touched is the fuel mixture adjustment, but I can't figure out how can it be set to run richer, as I have tested that the lambda control is working properly (around 40% +/-5% duty cycle at both idle and 2500 rpm) and so it adjusts the mixture itself.
Perhaps they set it way too high so the ECU doesn't have margin to compensate for so a rich mixture?

He told me that the car ran about 1 year that way, and that there wasn't any noticeable damage after it was re-lowered. This was about 4 years ago and now the car runs good and passes the emissions tests perfectly.

What are the potential dangers of setting the mixture too high? I can only think in the cat, but 1 year is time enough to have blown it, but it seems it hasn't.

What do you think, does it worth the pain re-setting it that way again? (leaving the emissions apart ;-)

Best regards