you might like to peruse these threads:
I'm in both of those threads (which I mention just so that you know I can back up what I'm saying with pictorial evidence).
Simply put, I routinely use the 100-400 and Kenko 1.4x (560mm of focal length) handheld, and I can't speak highly enough about the image quality of that lens.
Ignore any nonsense about the 100-400 not being capable of real sharpness - it's just not true.
This might whet your appetite:
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Yes, I was quite close to this lapwing - this is as near as dammit 100%, but it's still handheld at 560mm.
Now then, I'm not a big fella, and at 46 (closer to 47, actually) I'm not exactly in the first flush of youth. But I'm passably fit for my age, and I think nothing of 6 or more hours at a time with my 30D, grip, 100-400mm and TC around my neck, and a rucksack full of photographic bits and pieces, food and drink on my back.
I'll also point out that the IS in this lens will often more than
make up for the sharpness "advantage" the prime is supposed to have. This is at 1/320, which is waaaay lower than the 1/896 (1/FL * 1.6) that current wisdom says you need to handhold a FL of 560mm on a 1.6 crop camera...