I ran an experiment with adult BG in a 100 gallon trough. It took them 4 days to catch and eat 50 Gambusia, and less than half that time to catch and eat 50 adult FHM.
In a pond setting, a breeding population of FHM will, I believe, eventually be wiped out by BG - but it would take a lot longer than it would for LMB to take them all out.