1) There's plenty of information on the effectiveness of FSD shocks on Super C motorhomes - and probably on Class A motorhomes too ... a search of the WEB will lead to users comments on the various truck and motorhome blogs (as I discovered). FSD shocks have been available for the E350 for quite some time. The WEB probably has plenty of user inputs on them too. If push comes to shove, someone really interested in what the upcoming E450 FSD shocks might offer could contact the KONI company directly (as I did regarding availability).
2) Removing a leaf need not lower carrying capability. Air bags can replace any loss from a removed leaf spring on each side - if you get ones that can carry the poundage that the leaf spring was carrying. Of course air bags rugged enough to replace one leaf on each side can also restore, or increase, vehicle height. After doing this, you get to ride most of the time on air in the rear without spending the big bucks for full air suspension.
3) As I said, plenty of E450 shuttle buses have full air suspension systems. The cost is in the $4000-$5000 range. For folks who shell out the big bucks for a new E450 based Class C motorhome from $70,000 on up and really love their rigs ... maybe $4000-$5000 to make their E450 ride and handle like a sedan (but still carry full rated weight) would be for them money well spent.