Thunderbolt is the fastest connection of those that you have. As to the WD drive, do you know its rotational speed? You'll want 7200 rpm. If it's 5400, that will noticeably slow data transfer. Why not phone OWC and ask their advice?
You can read about the differences between versions of Thunderbolt here, Note that Thunderbolt 1, although the slowest of them, is still way faster than FW 800.
