The subframe mounts play a big part in determining the temperament of the rear suspension. It's called a kit in the aftermarket because it comes as a set of left and right bushings and a diff mount. The parts are available separately from the dealer.

I think springs only address the height of the car. The condition of the spring pads probably contribute more to ride quality than the condition of the springs.

Put it this way - I haven't heard anyone say, "I put new springs and the ride is so much better." Unless a spring was broken or it was part of a swap to/from a sport tuned suspension.

Sixto
95 S420
87 300SDL