Choosing the right suspension system depends on what your going after. Do you want adjustability for weekend track events or not? Also, you should focus heavily on what the SPRING RATES are with each kit. The higher they are the less body roll you will have but you will also get a really harsh ride, and if you have 19inch wheels WOW watch out. Spring Rate is the most important I think in choosing my future adjustable coliover kit. You will be suprised that you might not need such big swaybars if you go with a higher S.R.

 

From what I am hearing it sounds that the stock Zs suspension is already a bit over sprung(or under damped depending on how you look at it) Changing spring rates without the proper increase/decrease in damping can make things even worse. Also keep in mind that there are quite a few sources for linear rate springs in every spring rate, so if theres a coil over set-up you prefer that doesnt have the spring rate you desire you can simply replace them with different rate springs from a seperate supplier, look here for springs by rate/size http://www.pegasusautoracing.com/