The LS4 itself is lighter than a L67, fully dressed. The GXP is heavier curb weight also. The front springs are the SAME on a GXP vs many other 3800 platform cars. The damping is a different story, as are the control arms and sway bar (aside from differences of mounting points).
So, some feedback on the GXP Bilstein install on the GS, hopefully it will be useful for someone. I did the install yesterday with some help. First road impressions are astonishing - the car is absolutely transformed. The car had rather worn out FE3 struts in it (one was almost totally blown), so the difference is very noticeable. I will say that the car is not upset by road imperfections, and that the traction and responsiveness over all surfaces is greatly improved. There is more road vibration transmitted but it is not intrusive. I am running Eagle RS-A Pursuit tires so they have a stiff sidewall. Coupled with the new strut mounts and 9C1 lower control arms, I'm not surprised there is a bit more noise.
The Bilstein dampers are WORTH EVERY PENNY!
The car:
04 Regal GS
The new parts:
Bilstein GXP dampers at front and rear
KYB strut mounts
KYB front strut bearings
KYB strut mount hardware, insulators, etc.
Moog strut boots
Dorman 34mm front bar (control arm mounted)
Stock rear sway bar
9C1 front control arms
The reused parts:
Stock FE3 (TJA code) springs front and rear
Modifications I performed or parts left out:
Rear strut boots were a combo between Moog and OEM front boots (pics later)
All bump stops (new struts they are internal)
I used a single-action strut compressor for the rear components as the standard hook and bolt type don't fit. I recommend buying Harbor Freight item 43753. Link here:
http://www.harborfreight.com/single-action-strut-spring-compressor-43753.html if you are going to do this work. Very nice tool and cuts down the time considerably. Also, have someone help you. Safer and easier.
I took lots of pictures to provide some references for others but this forum has terrible attachment interface.