I think the answer to your question is that an eager zeroed thick disk gives the best performance. From the Performance Best Practices for VMware vSphere™ 5.0:
An eager-zeroed thick disk has all space allocated and zeroed out at the time of
creation. This increases the time it takes to create the disk, but results in the best performance,
even on the first write to each block.
For FT to work it needs to enable multi-writer mode on disks which are shared by the primary and secondary VM. Multi- writer mode can only be enabled on eagerzeroed disks... More info here: http://kb.vmware.com/kb/1034165