Aren't you guys afraid to mess up with engineers calculations on suspension modifications?
I would not worry about adding some plastic shims that will slightly adjust factory setting, but pushing the spring 2" before designated point can open whole can of warms. Would that spring bend past breaking point at some time?
Is it possible that add-on overload spring will hit the main springs and act like solid stop in suspension travel?
I've bee using airbags for decades and those not only avoid such concerns, but air makes much better cushion than solid blocks.