I have ruled out taking the 100-400 as it is too soft.
May be I'm quite biased about the 100-400, since it is a lens I adore. I'm afraid you get a bad example of it and that is a shame, since, I know that for personal experience in photosafari in SA, the 100-400 has the perfect zoom range for the business. By the way, what do you mean by 'soft'? Is the following image (100-400 at 300) soft for you? (Unfortunately I had to resize it to allow uploading it, loosing a lot of IQ)
P.S. I think the following article could be very useful.
I'm very well aware that the 100-400's are not all created equal. There are good ones out there and there are bad ones. I agree the focal length is awesome for a safari. But what made me stop using MY 100-400 altogether is when I tried to micro adjust it, the output was too soft that I wasn't even able to find the sharpest point. I do own the 70-300L also but I opted for the 70-200 f/2.8L II along with the 300mm f/2.8L II instead because I can put converters on both of these lenses, their faster and sharper.
As for TexPhoto's idea of zooming with feet, I'm just crazy enough to do that but I want to be able to shoot again the next day not being the days kill. I love a wide angle, I like getting right into my subjects face but they won't let me in Africa for some reason.