Hi Moriyama and Dauncey ,
Thanks for your valuable suggestions regarding this setup.
Now we are planning for migration of their existing setup
Older SOA gateway Nodes ---> 9.3 version of gateway
Developer portal 3.X---> 4.X version
The customer will add two more data centers with each cluster in each data center with existing data center. Portal 4.X will still support multiple clusters and they have limited use of portal . They are not using analytics, or monetization for today. I believe wrt your 2 point putting all nodes into a single cluster and spread them across data centers is not a good idea if the latency arises. So having 3 separate clusters for different data centers as we have done it for other customers.