It depends on what sort of applications you're running, how many concurrent
clients you want to serve, and how much competition FMs will have for the
available bandwidth (as in other servers, people in the office using the
internet connection, etc)

If your FMS apps are all data and no video, then your 1.5mbps will do nicely.
If you plan to serve video, you won't have much user capacity. Let's say you
want to serve a 500kbps vidoe stream. With your 1.5mbps pipe, you could
adequately serve 3 concurrent clients before you run out of bandwidth.