I have thought about running a common sump but probably won't - at least not for all my tanks. The reason is because right now I am running one system with zeovit, and so far it is going well, but I want to give this at least a year to see if I really want to keep on using this and if I do want to, am I prepared to use it on all my systems. The answer is probably not because I want to have different tanks and each will have different needs. I truly believe that a softie system actually does better with slightly higher nutrient level whereas a SPS system requires pristine, low nutrient water.
I don't think I will have to worry about trading up in skimmers - I will probably run the 190 and new 300 with a common sump and the skimmer I have on order will skim that and probably more.
If anything I will probably be selling one or two of my existing skimmers.