Bird -
It has been studied with institutional endowments several times. The studies show that 50% of the replacement funds under perform the replaced funds.
There are also several studies out there that look at skill vs luck of the investment manager (Eugene Fama's is one of the most recent). The studies find that only between 0.6% to 2% (depending on whose study) of the investment managers may be skilled, the rest may just be lucky.