It's going to be functionally impossible to get good data. Not least because I suspect the vc industry really really doesn't want potential startup employees to see the numbers. Or to think to hard about the wave of upcoming ipo devaluations coming to unicorns (viz a recent discussion from Mark Suster where 5/7 of 2015 large ipo exits where down rounds compared to previous valuations [1]). If the numbers were amazing you'd see someone like First Round giving exit surveys to all their companies and trumpeting the mean or median outcome. Or even YC; they are probably in a position to collect that data.
The best you'll be able to do is a site like glassdoor, with all the sample bias that implies. But even with glassdoor, I've told a recruiter to go away because their company pays poorly according to glassdoor. The recruiter then whined about glassdoor, but since he didn't provide me with salary numbers, what does he expect?
I've also pondered building a site similar to glassdoor to confidentially discuss outcomes, but the best you'll ever be able to do is anecdata.
[1] http://www.bothsidesofthetable.com/2015/10/18/venture-outloo...