I think you're totally failing to appreciate that
mobs dying faster increases your survivability. It's not simply a fact you'll stay alive more easily soul cleaving instead of casting SB. On a tanking dummy, while running at least 1 gaping maw, I'd guess you're right. In practice, I think you're totally off, and the data don't back you up.
As said, no one runs FD over spirit bomb:
https://www.warcraftlogs.com/zone/ra...spec=Vengeance
Moreover, look at the logs.
Here:
https://www.warcraftlogs.com/reports...mary&source=56
Soul cleave is 15% of healing done, Spirit bomb is #3 at 10%. And you say it's nothing? Come on. More importantly, spirit bomb is 5% of the group's dps, and that's for tyrannical without teeming. Mobs dying faster means you have less to surviving to do and healing percentage is going to be higher on fortified weeks.
The real answer is not to do the Corsairs in FH while you don't poison dispel. More soul cleave with gaping maw traits might help with that particular problem, but that's like putting on a helmet so you can bash your head into a brick wall. (The real answer is to not do it.)