The cheesy thing to do is to heat the springs with a torch until they sag to the height you want. That's a redneck "performance upgrade" that is not worth doing. Cutting springs with a cutoff wheel is fine, although the stock Z springs are WAAAY too soft so you'd do better to get a set of stiffer aftermarket springs. Also, in regards to the spring rate going up, Dan Baldwin did the calculation some years back and cutting a couple coils and added something like 5% to the spring rate. So it's not going to change enough to compensate for the car being so undersprung in the first place.