Thank you very much for all the detailed information, PJB and Gambaluga.
Now I start to get a clue about the shocks. It is easy to understand the principles of the raising spring rates but it is important to see the relevance of the matching shocks. I thought (but I was unsure about it) it makes sense not to change the back dampers as well as a must - especially if they are already tuned to the back leaf springs.
By chance my tin car is a W212 Merc. which has very good suspension and ride feel. May be that the following info is at least partly the reason why (I think it was one of the fírst cars equipped with it - paragraphs by autoworld.com) and it sounds similar:
The W212 comes standard with adaptive shock absorbers [passive direct controlled], which vary their damping rates based on driving conditions. In normal highway cruising mode, the shocks soften – allowing the car to absorb road undulations without troubling the occupants. During high-speed cornering or sudden directional changes, the shocks then firm up to give improved handling characteristics.
