It's not just the tire compound that determines the needed spring rate. It's total grip which is a combination of tires and track surface.
If you are running soft tires it is usually because the track is slick which means you need soft suspension as well. As traction comes up you may need to go to a harder tire. If traction really comes up (someone just sprayed VHT all over the track) then it's time to go harder on the springs as well.
That's just a bit of general theory. Every car/driver/track is a bit different.
Sean. Certified speed crazed mowron.
Team Shepherd USA