Probably being open from the beginning is the way to go, but if your top performers are going to leave because they hear layoffs are coming, they will still leave after layoffs have arrived. Most likely won't.
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.