What is your approach towards “productivity outliers” (both high and low performers) when it comes to optimizing headcount?

outliers do not represent your "normal" team's productivity. When you have high performers you need to spend some time understanding what makes them so efficient. most of the times is a skill or knowledge you dont have as part of your process, so you can't replicate it. Once you find the one or many differentiators you need to work with the high performers to embed that into the overall team process. share the knowledge, train your team.
on the other hand, a low performance has two sides: un-motivated or under-skilled. you need to identify what is the case. For under-skilled people, pair them with a mentor or a good performer to get them up to speed.
For a un-motivated employee, most of the times not even more money or recognition will make them excel. When people do the work because they are paid to do that and there is no passion or interest... hmmm... time to let them go.

Don't hire on forecast but instead on need and ARR.  Ensure conservative hiring practices for responsible scalability to avoid RIFs