I am also heading to Africa but in September.
Similar dilemma in what to take:
Canon 70-200mm F2.8 II with 2x tele conv
Canon 70-300mm as is but could take Kenko tele conv
Canon 100-400mm as is but could take tele conv
Canon 400mm 5.6
The 70-200mm delivers great IQ at 400mm with 2x conv effectively making 400 5.6, very similar to the 100-400mm at 400mm (as it struggles at 400mm) but obviously slower AF. Rules that out.
70-300mm small, good range but 300 I don't think is long enough, you could add keno extenders but heard mixed results.
100-400mm great all rounder, IS, but don't like the pump design its a dust trap and its pretty old, 400 is pretty poor performer which I'm guessing is where most will be shot.
400mm F5.6 L Prime, best performer very sharp. From reviews much sharper than any of the above, smallish, light and fast AF. 400mm cropped to 600mm outperforms the 150-600mm at 600mm but neck and neck at 400mm. Downside, no IS, not as flexible as its a prime would need to take a 70-200mm to fill.
Tamron - best range beats anything by 200mm, has IS, very flexible but large and heavy. 600mm is more like 565mm in tests, but from 500-600mm suffers from fairly bad CA and is quite soft. But it outperforms the 100-400mm from 100-400mm 400 is sharper across the frame than the 100-400mm at 400mm but the 100-400mm is sharper in the centre.http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=929&Camera=453&Sample=0&FLI=3&API=0&LensComp=113&CameraComp=453&SampleComp=0&FLIComp=7&APIComp=0
So basically the tammy is better than any zoom canon has to offer at this length and as its £200 cheaper you get an extra 200mm you can use for free. As there is nothing to compare it to at this length in a zoom or price range its a compromise worth taking. The Tammy also holds its own vs the 400mm F5.6 at 400mm its only slightly shaper at F5.6http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=929&Camera=453&Sample=0&FLI=3&API=0&LensComp=278&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=0
So in my mind its the Tammy vs the 400mm F5.6 not sure yet but leaning toward the Tammy.
At £950 the tammy is a bargain. Cheaper than all of the Canon offerings and holds its own to all of them.
The only concerns I have copy variation, haven't heard a lot about this yet, also build quality and weather sealing. Africa is a very harsh environment, I have been to the sahara and ruined my 17-85mm EF-s lens the zoom barrel was so full of grit and the lens full of dust and it never left the camera but obviously wasn't designed for such harsh environments.
Similar problems could be had with the 100-400mm with its dust pump, but the 400mm L, 70-200mm L and the 70-300mm L may fair better??
Im leaning on this as my kit to take.
40D Coupled with either 17-55mm 2.8 or 24-105mm F4
(my only crop camera, like the IQ and if I don't feel 600mm is enough on the 5DMKIII you get near enough 1000mm on APC with the tammy)
35mm F2 IS prime for low light and a nice length for landscape.
5DMKIII with Tamron 150-600mm