Regarding the FSD shock absorbers not being effective enough on some rigs.
I wonder if it is most beneficial only when you have a minimal amount of load-margin when fully loaded on a trip. A short 24' motor home built on an E450 chassis is going to bounce down the road no matter what you do with exception to removing a leaf spring or two. The same fully-loaded rig built on an E350, the ride will naturally be much softer and so the FSD shocks can be more effective.
We own a rig that is 23-8" long end-to-end. It is built on a 2007 E350 with a GVWR of 11,500. On our heaviest trip, it weighed in at 11,480 pounds, right at the chassis limit. The rig is equipped with the older red Koni-RV adjustable shocks set to the stiffest setting, along with HD stabilizer bars. During that trip, I laid on the hard floor face up with my head resting over the rear axle. I felt every road imperfection like crazy. I would hope the FSD shocks could soften up just that very thing. I wish I had a good reason to replace our good condition, working red Koni RV adjustable shocks, but we were not blessed with money to burn like that.