Other than weight and IS, the improvements from Version I to II was inconsequential. Two identical pictures with no shake most people would be hard pressed telling which was taken with which lens.
On the other end of the rig however, the difference between a 24mp body and the 5Ds R is very noticeable.
While the 6.71 lbs 600mm III with 5 stop IS would be help full, it would be a huge hit to the wallet to upgrade for a minuscule amount of improvement.
I would expect the 500mm to be the same if they decide to release a III version.
For me I want to see where the R goes, and possibly see some of the "rumored" supertele's that will go with it.