I don't use the Sigma 300=800 personally but I hear that it is a good lens and better than their 800mm prime - though more expensive.
I use the Canon 800F5.6 L IS and find that it is best used as a bare lens, it is OK with my Canon 1.4 Mk2 extender but not brilliant.
Some make an issue about the lack of IS/OS on the sigma. For what it's worth the IS on my Canon 800 is playing up (freezes occasionally) so, in January I turned it off. The net result of this was faster AF which I can live with! I don't know what camera you use (I use a 1DX) but so long as you can push the ISO a bit then the lack of stabilization should not be an issue.
If you cannot push to the Canon 600mm + lenses than the Sigma is the only quality option.