Thanks! That comparison is definitely helpful. I do have the 70-200mm f/2.8 IS II, so the results are close, but the 100-400 @ f/5.6 is still better at 400mm. However, with just a 1.4X extender the 70-200mm might even perform a little better. It also looks as though stopping down even a little helps quite a bit in bringing the extender combo closer. There should be plenty of available light, so I can probably afford to stop down to f/8 and still achieve a reasonable shutter speed for stopping action. Definitely looks like that tips the balance in favor of the extender combo.

Anyone with safari experience have thoughts? Is performance at 400mm the right thing to worry about? In practice will I actually end up zoomed in a little tighter on larger game subjects? (Or conversely, would I sometimes be wishing for an even longer lens?)