You could replace "employee performance" with "value to the company" and the same argument would hold. Performance is difficult to measure, but we get a good estimate of value to the company any time someone receives a competing offer and drags their manager to the negotiating table.

The amount of money the manager is willing to match is the perceived value to the company. This is how the company actually behaves (we know for sure whether they match the offer or not) and that behavior implies a value to the company, regardless of what anyone says in performance review season.

> The amount of money the manager is willing to match is the perceived value to the company.

This assumes the manager is irrelevant here. But we all know that different managers (or non-managers) can communicate value differently for the same employee. So this metric can't be solely measuring the value of the employee.