I've seen pictures of the sprint springs and while I don't have magic eye's that can measure wire diameter on sight, I just don't see that the wire in the springs is thick enough and the coil count's low enough to give out any real increase in spring rate vs oem. That being said, your problem is the drop amount and what happen's then you run out of avaliable shock piston travel. You kiss the bump stops which effectively spikes the spring rate giving the illusion of stiff spring rates. With that much drop height on oem based shock lengths, progressive spring rates will not save the day. You will be far better served with coilovers with suffecient stroke.

As other owners have likewise done, your more then welcome to sent me a single front and rear spring to have the actual spring rates determined. I would of course return them to you within 48hrs, shipped back at my expense of course.