Was not aware of the 100-400IS AF/IS shortcomings...certainly worth considering. I'd still take it. For shots from a vehicle on a game drive, having a zoom to quickly compose a shot is a huge advantage. Adding/swapping TC's on a prime, in a game car full of people, and dust, is slow, and a PITA to me. In that situation, a big zoom is tough to beat. I'm spoiled with my 200-400/f4. Just about as good a all around critter lens that I've found. Canon would be wise to copy/compete with it.
Jeff
