Do I need to deal with someone who hits their numbers but is making the team miserable?

Oblong's answer

Yes — and the fact that they hit target is exactly why this has probably gone unaddressed for too long. "Performance" isn't only output; it's output achieved in a way that doesn't quietly cost you everyone else's. Get specific about the behaviour, not the personality — what they do, in what situations, and what it costs the team — and have that conversation as directly as you would a missed target. Protecting a high performer's behaviour because the numbers are good is a decision, even if it doesn't feel like one.