[JacobLBryan]

One of the arguments regarding this is that employment in the sort of higher level careers we're talking about is fairly inelastic, so if you don't take the job then it will go to the next best candidate who applied. The impact is less about the total damage/good done by your position than it is about the difference between you and the next person in line (or basically the average person in that industry). If you're the sort of person who is concerned with maximizing impact in this way you'll probably do at least a bit better on the margin, and any money that you earn to donate is pure benefit since that wouldn't have happened otherwise.