The wait for the IS on the Canon must be a long time.
I use the older Sigma EX 50-500 DG (non-OS) with just a monopod and it works very well. If the new 50-500 OS is as good optically, it probably is a winner. The Canon EF 400mm f/5.6 is considered an excellent lens however so that is a lot to try to beat. However a 10x zoom is certainly more convenient than a prime lens though even if it is heavier.
The 500mm happens at infinity focus. As you focus closer, the focal length on the 50-500 reduces some so you might not get as tight of a view as you expect. I use a Canon EF 12 II or EF 25 II extension tube to restore the angle of view at close distances. This allows setting the focus ring back toward infinity and tightens up the view at 500mm quite a bit for closer subjects, perhaps almost as much as the 1.4x teleconverter but without as much loss of light and without losing autofocus.
Interesting. Have heard of people using extension tubes shooting birds. Something I plan to research and try.