For starters, Flake is not a Guy, but a Gal!
With Wildlife longer is generally better!
A 100-400 with a 1.4 converter will make it f8 at the long end - and won't AF on anything less than a 1Series.
A 120-300 sigma with a 2x may be a better option as long as you get a good one - The f4 will also end up at f8, but if you can spring for the f2.8, it will be f5.6 thoughout and will still AF on all cameras. But you don't get IS, which you wlll with the 100-400.
Going back to the150-500 Sigma, Fred Miranda users reviews give it a 8.5 overall.
http://www.fredmiranda.com/reviews/s...&cat=37&page=1
Would be a good option, but is slo at f/6.3 at the long end.