“Be a Superdonor” sounds like one is being encouraged to rack up a high score or become a superhero. That’s okay. And maybe it sublty would help people think about effectiveness. But it doesn’t put focus on the individuals people are helping. Any old local charity could ask us to be the same.
By contrast, what about “Be a Mega-Lifesaver”? This makes effective altruism out to be about the thrilling task of literally saving lives and life-years. That’s why I’m an EA. One problem is that this phrase is slightly more cumbersome.
You might consider italicizing the word “most” in the phrase “whichever organisations can most effectively use it”. This might guard somewhat against ineffective or complacent giving.