We run a gig LAN in our office but have a 100mb LAN extention to our hosting centre where I would like to put the secondary device.
Is 100mb fast enough for replication or is gig the only considered LAN speed these days?
This is a tough one to answer, as so many things can affect the Volume replication. Keep in mind it is Synchronous.
Consider the following:
1- How are the Xen servers connected to the storage? single path, bond, or MPIO
2- How fast are the RAID controllers?
3- How much data(changes) to the source volume is going to happen?

The reason I bring this up, is due to the way synchronous replication works. If the traffic is heavy(or the path is slow), IO on the source will be slowed down.
My suggestion is to test this scenario with the trial version.