Originally Posted by Lemans_Blue_M
I mean the RD Sport's progressive spring rate in the front will be much firmer (stiffer) than the softer Eibach springs in the rear.
That will hurt the overall balance & handling characteristics of the car.
This is exactly why all lowering springs are engineered and sold in matched sets
(front and rear 4-spring kits), instead of an assortment of 'mix-n-match' pieces.
Just playing devil's advocate here, but:
If you know what you are doing, and have a specific goal in mind, I don't have a problem mixing and matching. Spring retailers create a set-up based on what they think the average buyer will like. You may not be the average buyer.
I personally like a very stiff front end and a soft rear end (no comments on that please). When I ordered my springs from Ground-Control for the e46, I was able to set them up to my preferences.
However, if you don't know what you are doing, you can end up with a very strange handling car.