every asset class has a benchmark asset, and you measure the return against it.
the issue is that i don't know what asset class funds would place VC investments in. they could (but i doubt) benchmark against treasuries or whatever.
iirc a sufficiently good vc return is 3x over the life of the fund, which translates to an IRR of maybe 20%.
What do you mean by "high beta"? LPs are looking for uncorrelated returns. If you want high beta you could just make leveraged investments in an index.
For the rest: beta is defined as correlation to the market. Sigma is standard deviation of returns and also called risk.
A high beta asset goes up more than everything else when "markets" do well, and a low beta asset less so.
It is common in a multi-asset-class trading environment to make a grab for beta when the belief is that the short term trend for markets is good. Like when there is unexpected positive news flow about the global economy. "Buy some beta!"
My guess is that VC returns are dominated by IPO exits, and IPO exits need institutional and retail demand for equities, which tends to happen in up markets. To the extent that VC returns are directional with the state of capital markets broadly, it seems like calling it a high-beta strategy is reasonable.