I've been running Kirbans variable rate rear springs for years. No idea what the spring rate is, but it's definitely stiffer than stock. Really like it on the street.
I have a pair sitting in the garage with about 5,000 miles on them. Liked them a lot but changed up the whole suspension and they have sat on the shelf ever since. I would sell them for $25 plus shipping if you want to try them out.
I upgraded to variable rate springs in the rear years ago. Definitely firmer than stock. The more they compress, the harder they get to compress. In theory, it gives a smoother feel with smaller bumps but sharper jolts won't bottom out as easily.