I have a distributed application running on 40,000+ nodes (Linux) located at about 100 locations. We are on track to double both numbers over the next couple of years. I am evaluating Crossbar.io / authbahn / WAMP as a replacement to our current remote RPC implementation (ssh commands amazingly). RPC traffic is fairly light peaking at about 2,000 calls a minute across all nodes.
If anyone is involved in a project with similar scale using WAMP, please contact me off-list.