Spring Testing - Eibach Springs at The Tire Rack.
Lowering springs can improve handling, and the increased roll is offset by stiffer springs. I don't see a point to lower more than 1" though with just springs, in terms of performance.
Whether the rear needs camber kit depends on how much your car got affected by the drop. On my car for example, they were able to knock off 1 degree of negative camber to the rear without a camber arm, from -2.7 to -1.7, which is within factory specs.