There's a point at which health becomes more important than money. If someone is working so hard that the stress is taking such a toll that their marriage is at risk, they end up in a mental hospital, and/or are considering suicide, money ceases to become a consideration.

The most important thing for employers to consider in this sense is the wellbeing of their employees - this comes above everything else. If someone is healthy and happy, they can do a lot. If someone is stressed and unhappy, they're going to produce little.

And at the end of the day, the best way to improve performance is to ensure a proper work/life balance. It's about working smarter, not harder.