It is possible to produce extraordinary professional efforts over a sustained period—years in some cases. Sometimes—war, for example, or the merger of your company, or when a family member is threatened—such an effort may be justified.
However, the problem with operating at maximum capability is that the individual then cannot accept, or has difficulty accepting, emergent or emergency taskings.
Not only will his interpersonal relationships suffer, he is not reliable, for he will stumble over the most elementary additional tasks and he will choose not to take on a job he should. Because that performance will be out of character, his boss will be unable to count on him.
A person operating near one hundred percent capacity is a person operating at the edge of his envelope of reliability. He will not have the ability to take a bullet—of any caliber."
RADM Dave Oliver in his book LEAD ON.