the size of Berkshire Hathaway is effecting Warren's performance, he has a much smaller number of choices compares to 40 years ago. Berkshire still outperform the market in 1, 3, 5, 10, 15 years periods.
The monkey example you talked about is similar to the example he gave in a 1984 argument against efficient market hypothesis, search The Superinvestors of Graham-and-Doddsville if you are interested :)
