I don't know what the spring rates are for the Eibachs but, in general, if you install stiffer springs with OEM shocks the ride is going to suffer...the OEM shocks are not matched with the higher spring rate. There's not enough rebound. The ride is going to be bouncy...kind of like when you see those old cars driving around with worn shocks...they bounce like crazy over every bump or dip in the road...springs will not wear but shocks will.

I went through this with my M3...I first installed H&Rs and kept the OEM shocks...the ride was absolutely horrible. Every bump felt magnified 100x. Plus, the car no longer felt connected to the road...It felt like it floated...no feel. I eventually installed SA Koni Sports and it was night and day...the ride was soooooooo much better....