A common solution to conserve bandwidth is to use compression. This can be done easily in PostgreSQL by using ssh tunnels and turning on compression. I wonder why they didn't try that.
You see this kind of misunderstanding commonly pooled with other "a computer has physically moving parts" misunderstandings like the ORM or connection pooling concerns outlined.
After tyranny of abstractions, nobody knows how the moving parts really work.
// As alternatives given you'd like to keep the immutable data approach which brings a lot of goodness, consider a log-structured file system for the disk concerns, and geo-sensible replication for the latency concerns. At this scale, for near real-time app, bi-coastal DB is a bad model. You shouldn't have all users in SF querying a database in DC. Given the nature of the business model, they can share geographically at one time scale, and roll up and replicate geo diverse data at a high latency leisure.