the way to effective charitable donation is to pick the most effective charity and donate your entire charity budget to it. Thus, the only appropriate donations to SIAI would be nothing or everything.

Your conclusion doesn't follow from the premise. A small amount of money could reasonably be Plasmon's entire charity budget; when you say "nothing or everything" you do not qualify it with "of your charity budget".

edit: Oy, if I'd scrolled down!