So I know a few people are looking to lower an STS but nothing is available. I looked over my buddies CTS-V2 and ever thing looked inter changeable to me. I have an 05 sts 4 with mag ride so your results may be a bit different.
I looked at what spring rates were available and decided to go with eibach CTS springs for Gen2 3.6. for the most part every thing went in as if it were made for the STS. Now there are multiple paths that can be taken depending on what you want I will cover what I know.
With the springs I used the car will sit low. I started with a front fender height of 28 1/2 and rear of 29 1/8. I went through a few stages. first with the springs in stalled and no modifications to the ride height sensors I ended up at 26 3/8 front and 28 3/4 rear. So the front dropped 2 1/8 and the rear 3/8 lol Damn air bags. I unhooked the rear ride sensors and zip tied them all the down. This makes the car think the back end is high and will deflate the bags to a minimum pressure. the rear droped to 27 1/2 or 1/5/8" drop. not bad but still above the front. I then cut one of the lower coils from the rear eibach springs and that got me to 26 3/4. Its low for sure but, even now. should Look great On 20's
Next I needed to re work my ride height sensor rods. based on the motion ratio one would need to lengthen them about 1/4" per 1" drop. so I added 1/2". (5.5") I popped off the plastic ball cups and used 1/4" threaded rod to make new rods, it cost me $1.49
The car rides fine its not bouncy but it is a bit more harsh on pot holes,rapid bumps and such. The body is well controlled I have no complaints.
In theory if a person wanted a 1" front drop and wanted to level the rear you could use either a stock cts front springs and either eibach rears with 1/4" longer rod or cut 3/4-1 coil from the stock cts rears. I would think ctsv springs would be a bit stiffer and better suited to the weight of the sts.
In all there are options to STS owners who want to lower there sts, it might be little work but hell the springs might be available for almost nothing.
good cts spring info here http://www.cadillacowners.com/forum/...ibach-Engineer
The only info I could find sts spring rates, non v models are said to be 10-20% softer. http://www.topspeed.com/cars/cadilla...v-ar11514.html
Good luck Guys I hope this helps the guys that want to drop there STS.