I know its not a Z, but I rate the RS-R Down on my GRY. Its just perfect - no impacts on ride quality, just nicer, firmer ride. With my fitment, the increase is 40% from the stock 2.8kg/mm.
Z ones seem to be really well balanced for a heavier car, 10.2kg/mm front and 8.1kg/mm rears. Crazy not...