So is this part of the reason why Zuck would have framed FB's impending actions as being focused on low-performers? That way, people who believe themselves to be high-performers wouldn't bail.
It also depends on middle managers identifying the actual low-performers and not marking for removal high performers who they don't like or might take their job in a slimmed down org.
The overlap between people who think very highly of themselves and your top performers may not be 100%. It's probably quite a bit less.