I've heard good stories about the Sigma 50-500.
http://www.pixel-peeper.com for examples. Unfortunately no photo's of the 500D AND the 50-500.
The 50-500mm is an
amazing lens for its range, and good at the long end, but it won't be as sharp as the 400 primes, and you'll lose even more light at the long end (f/6.3 AIR.) For its price, it's a very good choice, though I don't know how it stacks up against the 100-400 (it's better than the Nikon 80-400 at the long end.)
Shooting a lot of wildlife, I can say that if the OP is going to do lots of wildlife, being able to crop a sharp image is going to be a huge benefit. As a wildlife shooter, I can say that 998 times out of 1000, you'll be at the maximum focal length of any lens, and with the two remaining times, in one of them, backing up will probably get you a better shot anyway- so the only reason to get a zoom is if (a) you plan a lot of non-wildlife photography. or (b) you can't afford a prime, but the 400/5.6 makes that moot.
Finally, every stop of light you can get gives you ~20-45m of extra shooting time at both ends of the day when wildlife is most active.
The 70-200 with a 2x TC actually makes the 100-400 look
good:
http://www.luminous-landscape.com/reviews/lenses/400v400.shtml
Until you compare the 100-400 to the 400/5.6
http://luminous-landscape.com/reviews/lenses/forgotten-400.shtml
I've never been a fan of 2x TCs, and generally there are very few times when i'll even drag out the 1.7x I have (I shoot Nikon) because it's just on the edge of what I deep acceptable sharpness-wise. I'll use a 1.4x when I have to, but especially for birds, the sharper the better because unless it's a huge bird close in, or an "environmental portrait" type shot, you're likely going to want to crop and sharpness rules there. There are people who find the IQ of a 2x TC acceptable- and for them, that's fine- for me (with a Gitzo 1548 and full-sized Wimberly head, so it's not a support issue) it's just not up to what I want to produce image-wise.
The OP should look at the images in both Luminous Landscape articles, by IQ results alone, the choice is glaringly obvious.
Paul