I have made comparative shots of with my Canon 70-200 f/2.8 with a 2x teleconverter and my Sigma 50-500. The shots from the Sigma were noticeably sharper. On an earlier test I compared the Sigma 50-500 and a Canon 100-400 and the results were almost identical for both lenses. I do not believe the 70-200 with a 2x teleconverter will give as good results as the 100-400.
When I compared, I chose the 50-500 because optical quality was about the same and the 50-500 has a longer focal length. I do almost all my wildlife shooting using a tripod or a window pod. If you are going to be shooting mostly hand held I would recommend the Canon 100-400 because of the image stabilzation.
