I have the GC kit on my car. I'm quite happy with it. I have Koni SA's too which I think make it more firm than my springs. My fronts are 500# and the rears are 150# straight rate springs. Mine is set up for autocross not so much street, but Sam could certainly help you selected the best for you.
I got my springs and GC kit from Sam Strano >>>>>>>>>>>>>>>>>
I change my rear ride height every time I go to the drag strip to help clearance my 27" tires, then lower it to drive on the street.
Hard to install is relative. It was not hard for me, but I have heard that only a someone who who is a install god can do it. The lower u go the more rate u need. If you go with more than stock rate you should get konis to dampen the springs. Most shock do not work well with high rate springs.