The 300mm 2.8 would be a sweet lens. Although most likely not enough reach for me straight out at 300mm if I was doing more nature photography. I tried using a 1.4x telexetender on my 100-400mm but it was too slow and difficult to autofocus.... the 2.8 would make it much better (and 420mm at f/4 vs. 400 at f/5.6).... Or better yet the 2x with 600mm at f/5.6