Does Sunken Temple work on a unit that is already 5 might?

Yes, Sunken Temple works on a unit that is already 5 Might.

According to Rule 708, a unit "is Mighty" as long as its Might is 5 or greater. Because the unit already meets the requirement of having 5+ Might at the moment you conquer the battlefield, it qualifies as a "Mighty unit" for the purpose of Sunken Temple's triggered ability.