Spring rates
Back in 11/01, I had a new set of custom-made coil springs done by CSS. I sent them an OEM front and rear sample. Dave, the engineer there, told me that the OEM fronts were at 111, and the rears were 90. I told him I wanted a 37% increase in the spring rate with a .25" drop in ride height as well. He said he couldn't do a progressive rate in the front due to engineering reasons, but was able to do so for the new rears.
According to Dave, the front increased from 111 to 148, and my rears went from 90 to the progressive rate variation of 92-123. I had this done at 17,750 miles, and now have over 40K miles. I've got no problems.
-Mitch