The spring rate doesn't change ride quality nearly as much as the valving of the shocks. Most of the shock setups for the Slingshot run around 225-250# front springs and 300-350# rears. We played with up to 550# springs in the front and the ride quality was still pretty good, the harshness most people feel is due to valving, not spring rates. Another case of that was when we worked to develop the Ohlins setups for the Miata's many moons ago. We had spring rates on that setup that were over 3 times stiffer than stock, but the ride quality was actually better than stock. Shock valving can make or break the ride quality and performance much more than spring rate.
interesting - always appreciate your input on such things - - - Thanks