GiveWell spends a lot of time making estimating their performance easier (nearly everything possible is transparent, "mistakes" tab prominently displayed on the website, etc.). And I know some people take their raw material (conversations, etc.) and come to fairly different conclusions based on different values. GiveWell also solicits external reviews.

I think this is as good of an incentive structure as we're going to get

I think it would be better with more competitors in the same space keeping each other honest.