If they're so great, please show us some returns vis a your competitors and other asset classed with similar risk profiles.
(2009 - 2013 figures from here: http://en.wikipedia.org/wiki/S%26P_500#Total_annual_returns)
You can't look at venture averages because the best firms are easy to identify and perform much better than average.
On the other hand, I would strongly suspect that the top VC firms massively outperform the VC industry as a whole, due to any number of factors, including deal access, ability to secure favorable terms, ability to make new rounds or exits happen, etc. In time, a16z's longitudinal performance may well beat the market. But it's way too early to call the ball.
I agree that you can't really look at the aggregate performance of the entire VC industry. It's probably a highly skewed distribution, with almost all the big returns going to a handful of funds.