I heard of a guy who tuned his already 500 hp Benz to get it over 600 (now he is under arrest in my country for street racing).

They also do this a lot with BMWs, e.g. the 318i and 320i engine is the same, so if you just change/hack some software setting on the 318i, you can get more hp out of the same car.

But these are of course the minority of people, and mostly targeting sport cars, not luxury sedans.