That may be on older K Bodies. On 2006+ DTS the coil springs # rate are exactly the same front and rear on a VIN 9 or VIN Y DTS. The only difference is that on VIN 9 cars the rear coil springs are a 1" to ½" taller to accommodate the MRC shocks.
Before trading in my Lux II I looked at getting the coil springs from a Performance sedan but then found out they are the same. When I swapped out the AC Delcos for Monroes the car rode a whole lot better. But MRC is really a lot better but even with MRC I find the car still float a bit. Par for the course I guess.
If you haven't tried it already... run 36 psi cold in the front tires and 34 psi cold in the rear tires.
The higher pressure pretty well eliminates the float on mine on the highway.