If I give 10% of a larger salary, fewer people suffer - so that's good. But of course, a better outcome would result if I both pursue a higher-paying job and donate a higher percentage.

This is exactly it, and it's something that people seem to misunderstand about Singer. He is merely stating that if donating more money will help, then it's obligatory.

In the event that spending your money in other ways will help more, or that working so hard will burn you out, then the antecedent doesn't hold.