John
Progressive puts out a good product that work well too. Have you tried adjusting the preload on the springs? On the WG it is the collar that you turn with a spanner that is made for it or you can grab it firmly and twist with some results. There are a few settings on the shocks, can't remember if it is 3 or 5 settings on that cam adjuster. The one that compresses the shock the most would be able to carry more weight better.
As far as service interval, some ride the bike and never change the shocks. Others switch out for a better ride with the use of a better shock in the beginning.
You can be the best judge of the condition of your shocks. If they are bottoming out and didn't do it before, it sounds like it's time.
