The springs are not going to determine the amount of "squat". The geometery set in the upper and lower control arms determine the amount of Antisquat and where your Instant Center is. The shocks will play a huge part in how much the rear of the car moves up and down. I have AFCO springs, stock style, and they are pretty awesome...225lb rate. I did spend some big coin on my AFCO double adjustable rear shocks. Being able to adjust the compression and extenstion independentable makes a huge difference. Take away some extenstion and the car will squat a little more but you don't want too much. I have mine pretty neutal right now, the extenstion is on 4 and my compression is on 3. I am going to the track tomorrow night and going to try some new settings that I came up with but I am leaving my control arms alone.
One thing that is nice about rear coilovers is the ability to lower and raise the car immediately. Doing this you can adjust your IS location and your AS figure just with a few turns. And that will also help you if you decide to get the car scaled.