You need some preload on the springs or they will unseat if the shocks become fully extended. If you want to go lower, you need different springs, shocks or, as Julian suggested, helper springs and spring guides.
Also, the spring rates strike me as a bit odd. The springs support the weight of the car, so the rates should reflect the normal F/R weight distribution. You shouldn't be using springs to tune the ride firmness or handling characteristics. That's the job of the shock absorbers.