The obvious first step would be to see if you can bump that internet speed up with the ISP.
As for 50 concurrent WD users + iOS users, you'll never have a great experience with < 1mbps upload. I'd consider a hosting company at that point.
Mirrorsync is a good product for syncing multiple instances of filemaker server. That might be your gateway to WD later on.
For the time being, the .8mbps upload is a bit of a bottleneck, and it is a two way bottleneck for what you've proposed. So the decision should lie with what group is considered the primary users. If it's the FMP desktop users, put the server in the office. If it's the iOS users, put it in the cloud.