There's a brief but interesting article in Bimmer #101 (Oct2011). The author spent some time talking to TC Kline about his philosophy of suspension setups and got to drive his modded (suspension only) 128i. Briefly, your spring rate must be dictated by the grip of your tires, go with a less stiff spring than you might think, and add more control with a robust damper. You "must" change rear subframe bushings to keep up with uprated springs. Check it out!