Here is a good link about lowering and roll centers:
http://e30m3performance.com/myths/We..._transfer2.htm
No one here has mentioned it, but the spindles do increase suspension travel. The problem is the tire will now be touching the top of the fenders! If you have regular shocks, this may be an improvement. The RS kits have the shock rods shortened at least two inches so they can be used at super low heights.
The RS kit can use firm springs but the ride is excellent for this type of product. Lower rates springs are easily swapped in if you prefer more ride comfort. The Koni and Bilstein shocks are really harsh by comparison. You can change springs in an hours time max. Plus you can again change the balance of the car by changing spring rates front to back.
I'll stop talking or people will think I'm on the payroll of RS Racing!