I took the car back and told them to test the thermostat to make sure it was not defective and to reflush the radiator in case they dislodged some gunk and didn't get it out.

Naturally they weren't going to do any extra work, so they told me they did "the research" and found that Mercedes now specifies an 87C thermostat (instead of 80) because if the engine runs hotter it is better for emissions.

Sounds like baloney to me, or at least that I should have been informed to make my own choice if I wanted a higher temp thermostat or not.