thats because a grand prix isnt meant to be lowered. it throws off axle geometry (causing vibration while accelerating), makes the car ride really rough, and parts wear out much faster.
the best springs for a slight lowering and still a good ride are vogtland springs.
the best springs for slamming the car are intrax, but the ride suffers immensely.
the best springs for an in between look are either SSC, B&G, or eibach.
the overall best setup would be coilovers ($$$) or baggin it ($$$$$).
Ssc's are garbage!!!