My sibling deserves full credit for thinking of and writing up this post but he doesn't frequent the EA Forum so asked me to post it.

How can we assess the impact of money that is given away immediately versus money that is given to a well-managed endowment to be paid out in perpetuity (paid out at a rate of, say, 5% / year, compounding at at least 8-10% / year)? Are there well-managed EA endowments to support?