From what I've heard about Eibach, it lowers your car just enough so that ride quality, handling and front end geom. are not compromised. Thus, eliminating the need for adjustable C/C plates.
Difference between Eibach Progressive and say...Steeda Sport: Eibach had a fixed rate front; somewhere around 600lbs and a progressive rate rear; somewhere around 320lbs. Steeda Sport is fixed all the way around making the car lower and comprimising ride quality.
Also if you want to have passengers in the car with you, you might want to consider Eibach Pro's because they won't drop when you have people in the car.
My choice when I get the money will be the Steeda Sport with their C/C plates to fix the front end geom.
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Green '94 GT conv. -- K&N filter charger, FMS 9mm wires, Accel hipo cap/rotor, 180* thermo, 14* timing, off road H-pipe in front of 2 chamber flow's, Steeda subframe connectors w/ crossbrace, and still workin' on the rest!
My 'Stang