Air Bags or Roadmaster Active Suspension -Help

50% of what number? I don't see on the Roadmaster site where they talk about any values - likely because it is too committing.

If I were going to modify the springs I would add an additional leaf, not a small coil that pulls on 1 side of the leaf only. I have airbags - I know what they can do and highly recommend them for applications such as the OP is requesting.