If you have the sport suspension, you have them. Did you get the base model (1SA), the Luxury model (1SB), or the Luxury Sport model (1SC)? The sport suspension also comes as a free flow option (RPO Code is PDX) in 2005.
Also, if you have the Xenon HID headlights (they look blue instead of yellow) then you also have the Nivomats, so you don't blind oncoming traffic when you have your 3 linebacker friends in the back seat.
Below is what they look like. Stock CTS (sport model) on the bottom with the blue stripes, the CTS-V version is on top (with the WildWhl lowering mod already done), with the pink/purple stripe. I'm not sure what color stripe is on the Luxury or base model shocks (green?).
If you have the sport suspension, you will have issues. Eibach recommends that you swap to non-load leveling shocks. I upgraded to V shocks all around (the V guys usually upgrade to the stiffer FG2's and will usually let their FE4's go for under $100/set).
The V Nivomat differs from the CTS Nivomat in the upper hat area. The V can be disassembled, the spacer removed, and new spacers inserted to lower the top pillow mount. On the regular CTS, the plastic shock tube, spacer, and top pillow mount are one piece. I've been meaning to look at my old FE3's (stock CTS Nivomats) to see if the spacer will come out from inside the tube, so they can be lowered.
I don't think anyone has done the WildWhl mod (to install lowering springs) on the stock FE3 Nivomats. Members have either gone to the non-leveling CTS shocks, or upgraded to the V shocks. Incidentally, the CTS-V tube/spacer/top pillow mount will interchange with the CTS version, but you can't get them without the V shock, unfortunately.
My rational, after much research, was that the Eibachs are within a few percent of the stock V spring rates, so why not use stock V shocks?
I think the rear non-load leveling CTS shocks are pretty cheap ($64 each at GMPD), too. Part number is 25758571 according to the CTS Parts list (available at CadillacFAQ.com). I think Marine runs these. Check out the ride height of his car for reference over on Cardomain.
The V shocks do ride much more firm. I loved the stock Sport suspension ride, but the V shocks and Eibachs are much more firm and sporty. The WildWhl mod (http://www.cadillacfaq.com/faq/answers/lowercts/index.html) only lowers the rear about 0.8" to 1", so if you want a level ride, you'll need to shim the fronts up with a quarter inch spacer also (I still have a spare set I can send you if you're serious), since the Eibachs advertised drop is 1.2".
Look and see if this is low enough (pic links below). If you want lower (like Marines) I'd go with the non-load leveling shocks as Eibach recommends.