@nn: its a logical setup I think. The heavy leaf pack is there to essentially carry the load where it can easily carry more than even very stiff coils. The added coils would come in essentially when the leaf pack bottoms out. On very rough stretches and especially where you have to make a run up bumpy inclines, the coils would act like air bumps on a rally car, preventing/softening the frame hitting the hard rubber-bumps and/or the shocks from possibly bottoming out....
And yes, if one is smart enough to get the spring rates right, you could possibly create a kickass cocktail rally suspension based on this setup