Exactly. The only way to judge if a person is overpaid in any profession is by performance. It doesn't always work that way, but that's the best way to measure if someone is worth what they make.
This is the wrong question. CEO's are probably no more overpaid than many other people who can fool others into paying what they ask. "The worker is worth his hire." They are however wrongly paid. Their compensation should be subject to proxy fight every year, which keeps stockholders aware and interested. Labor, capital, managemant; each always thinks the others get too large a piece of the pie.