We'll be using a DNS monitoring service that would update queries to the new external IP's.
This statement leads me to believe that you'll be using different external facing IP addresses at datacenter B than at datacenter A in the event of a failover. Since your objects are going to be different (at least your virtual server IP addresses), I'm not sure sync is going to meet your needs.
If you are going to be using the same external facing addresses for your virtual servers at each datacenter, then not only DNS, but also routing would need to be modified to use datacenter B in the event of a failure at datacenter A. However in this scenario, a sync-only group should work.