If they are like other progressive shocks, the spring rates should be (stamped or embossed) in small numbers on the coil spring. There should be two numbers. Those numbers can be (or used to be) researched on Progressive's website, or call them and ask for tech assistance and ask. Whether air or oil shocks, the springs will continue to function even if the air or oil are leaky or defective. It sounds like your springs are on the heavy duty side. I do not know if Progressive will rebuild 416s, but even if they don't they may agree to change the springs in them (to lower rates).
http://www.promecha.com.au/springs_basics.htmhttps://www.progressivesuspension.com/c/tech-info-springs