Have you considered this route?
http://offroaddesign.com/catalog/shacklekit.htm
A flatter spring will flex better than an arched spring giving you a better ride. Depending on the kind of loads you are carrying you might be able to remove a spring or two out of the pack to soften it up a little. I took 3 springs out of my pack, but haven't got it out of the shop yet to give you any real feedback yet.