I got back to my modelling app Vectorworks and set the mapping there at the right scale. Then exported to Cinema again and all was well. Although I got half broken previews, dark and shiny, after a while turned ok again. But it's rather a hassle to work with without previews. Trying to adjust scaling of mappings with the material tags is very buggy and leads to unpredictable results apart from now and then locking up C4D/CR completely.
The main problem is that I don't know how you can prepare a displacement image like a jpg to scale in Photoshop to have it match geometry in C4D. Do the mm matter or is it just the pixels set at image size in PS. I noticed that scaling mappings in C4D using the mat. tags is seriously buggy for displacements in Coronarender especially above higher scaling factors. This is I think because the material tag scaling is not kept up to date with the hight and depth of the displacement. So at 500% upscaled tags the displacement also needs to go up like 5 times. It all leads up to a mess quit often. This is a problem for me in Archviz using masonry, wooden boards and anything detailed and small in facades. A lot of time gets waisted there. Maybe even most.