The FE2 rear shocks from a later CTS will have more closer matched damping to the FE3 self-leveling nivomats, than the soft ride FE1.
The nivomats are self contained hydraulic self leveling shocks. No electrical pigtails. Just R&R with non load leveling shocks.
GMPartsDirect has them for around $95 each plus S/H, which usually runs around 15 to 20%. List is $160 each.
shock absorber, cts, w/o sport suspension, ride and handling susp.
04-07 $159.73
$94.69
My car is probably not representative of a stock CTS with Eibachs. I lowered it after the V body work and 18's. I've got FE4's with the spacer mod to the rear to drop it a total of 1". I shimmed the fronts up 6mm (1/4") to level the whole car. I also have 245/45R18's on it (about 1" taller than the stock 225/50R17's). A stock CTS with FE1 or FE2's and Eibach springs will sit somewhere between 1/4" and 1/2" lower than this pic. I think my springs have settled slightly (maybe 1/4") since this pic was taken shortly after the Eibachs were installed.