Back in the day, I redid the suspension on my daughter's 760 Turbo wagon. The Nivomats on the rear had given up the ghost. She was in a band and hauling guitars/amps was the order of the day. Put in Bilstein touring shocks/strut inserts and IPD's overload springs in the rear. Since the Nivomat contributed to spring rate, the stock springs used with those would have been way too soft.

That combo worked beautifully - especially when loaded. A bit firmer when unloaded. However, unless you go to some air-adjustable device, you can't have your cake and eat it too. Firmer springs to help deal with heavy loads are going to ride firmer when unloaded. If you go that way, suggest removing the rear sway bar if the car has one, and/or going to a larger front sway bar. Of course, tired bushings front/rear should be replaced.

By the way, the Koni Sports have a LOT more damping than the Bilstein HD's....