The problem is there are no progressive shocks as shocks are time related [inch per second] where as progressive spring get stiffer [or have more rebound energy] the more they compress.
Adding a non linear element [springs] makes it MORE impossible to select a shock that works under every condition.
Do you select rebound stiffnesss for the soft or hard part of the spring or the mid point..........either way it will be wrong.
That's the problem with Eibach rear and Tokico blue shocks, not enough rebound when the shock gets 3" compressed and too much in normal street driving.
In many ways vehicles ARE progressive if they have rubber bushings, as the first inch [of sway or movement] is compressing rubber which is in series with springs, shocks, sway bars.........once the rubber is compressed the almost true linearity begins.
Same with tires [they deflect]: a 1500 pound [sidewall stiffness] tire in series with a 150 pound spring, sitting in a rubber seat.... with a parallel shock and sway bay each with there own rubber isolation.
http://www.caam.rice.edu/~caam211/hw12/ ... up...r.pdf
But a 100>120>135>150>170>187>200 pounds per inch [in half inch increments] is not as bad as it could be.