The GCs with stock shocks will IMMEDIATELY ride like crap because they do not have enough dampening to control the significantly increased spring rate.
Get some shocks with good controllable rebound dampening to use with the GCs. I have GCs and AGXs on my P10 and they have held up well for about a year.
As for ride quality, that is a subjective thing. I have lowered my car with the GC/AGX set up from anywhere from 1" to 2". The ride quality is about the same.
IMO, you get poor ride quality when your car bottoms out. That is why running 300 - 350 lb/in GC springs results in ride quality that is better than regular Eibachs. Because regular Eibachs lower your car but does not firm up the ride enough and the car bottoms out. The difference is, the regular Eibachs will ride smoother over small ripples and bumps, but when you hit a moderate sized bump or go over railroad tracks, the Eibachs will bottom out badly and ride like crap while the 300+ lb/in GC coilover set up (with the appropriate shock) will feel almost like stock over this rougher stuff.
Glen