You can use just the springs, the panhard bar is advised the recenter the rear under the car, it gets offset to the side when lowering with the factory bar. Any panhard bar equipped car is going to shift when lowering the car, in a way a panhard bar is a pendulum…
Lowering with just the springs will cause excessive camber and uneven tire wear up front, that is why we use our strut mounts, in addition to being stronger they have room for camber adjustment, the stock ones do not.
Camber bolts are a cheap way of trying to get around using strut mounts, they have less room for adjustment (about ½ degree vs 1-1.5 degrees with mounts) and are an eccentric bolt.
The dampers (shocks and struts) control how the springs oscillate, the factory ones are matched to the factory springs. Our springs will work well with the factory ones, but upgrading is definitely worth it in terms of ride quality and performance both.
Springs, dampers, and mounts all have the same labor so doing them at the same time also saves on labor cost/time and alignments.
If you would like to talk off - line ... you can send me an email at tim @steeda.com.