I'm curious as to why anyone would want to mess with that cars springs in the first place and the question was "lowering" springs BUT the person you mention posts at Swedespeeds R forum, not from the US and I do not see this topic posted there.

As for adding stiffness, like stiffer springs, IF you've driven an R in it's most active mode (one of 3 electronically switched on the dash), you'ld know that it's so stiff that no one would really even want to drive that car on regular roads like that as all the roads imperfections would drive you nuts. It's very harsh and done with the push of a button, no real need to swap out good parts IMHO.