FMA is now know as the Cloud Tiering Appliance (CTA) does not support VNXe as a source only a target.  Therefore the use of CTA in your proposal is not possible.  What is you ultimate use case backup/restore or archive?  If you can please provide information on the infrastructure.  For example what is the number of remote sites? Are the centeras already deployed in the data center?  When deploying ROBO it is easier to design the entire solution with a blank sheet of paper from the data center out.  In some cases (I am guessing yours) you already have infrastructure that you want to leverage and it makes it difficult to identify remote infrastructure that can be used to meet your requirements.  I would be great to continue this discuss in this thread, but if you feel more comfortable you can contact me directly.  -m
we thinking about deploying VNXe to some of our branch offices and 14TB of SAS drives and all the data will be replicated via FMA down to our head office data centre to our centera.   Do you see this as a viable solution? as alot of our branch offices all their data are just out of control hosted on their file server and they run out of storage. 
VNXe with FMA