So... we unfortunately haven't used SM for all of clients.. enough to run into a "storage" issue of data (if you will).
But we do a large amount of SaaS .. where storage is an issue... and traffic against that stored data.
For us, the real question would be .. when does it make sense to have SM spread over more than one server.. and if that is an issue.. how does one control it? So in a word clustering .. and control of data.
Doesn't SM17 or 16x has where different domains can have their email stored in different locations??
If that is true, and we were in your position.. we would just add more storage arrays.
Be interested to see how others are handling volume and data storage.