Today I spoke at some length to two of the UK's mail-order film suppliers. They report that despite decent supply of the RVP 100, the original RVP 50 is out-selling the new film by a great margin. Further they both indicated feedback from users of poor shadow detail and high contrast from Velvia 100. I could get both of those simply by pushing RVP 50.
Both companies indicated that they were still -so far- getting supplies of the original Velvia from Fuji, and I feel that were Fuji convinced by their current Velvia 100 they'd be working very hard to complete the conversion process. Personally I still regard RVP 50 as a mainstream film and I'll just keep trying a few rolls of Velvia 100 until I'm convinced they have it right.