Thanks guys, this is super helpful information. Those are some really nice shots, BTW.
Looks like renting the 300mm f/2.8 may be the way to go, bringing an extender for some extra flexibility. Will keep the 24-105 f/4 for general purpose use and reluctantly leave my favorite lens (70-200mm f/2.8 IS II) behind this time.
We're going to both the Serengeti and Ngorongoro. Is it worth trying to find a way to cram in a wide-angle lens as well (I'm thinking 16-35mm) for landscapes, or are there not enough opportunities to make it worthwhile.