At any given moment, most companies can probably justify cutting 10-15% of dead weight. But when you cross that line, that's a much different smell. The most talented get a whiff, update their CVs and head for the door.
The goal is to cut the bottom and leave the top. Take off too much bottom and your top will leave "voluntarily".
> 10-15% seems to be the number that reflects “big enough that I can tell myself and my investors or board I am doing it, but not so big that it causes truly uncomfortable conversations for the team”
Most people don't find that such a terrible idea.
The 10-15% percent aren't necessarily all bad performers but might just be people working on teams that are to be disbanded entirely.
Team that managed to hire good people and fire bad performers already ? You're basically taking productivity out.
Ops team that every other software team is dependent on and already overloaded ? Now every other team will feel delay in dealing with their stuff.
The next step is to discuss the usefulness and your own perspective, not to mock.