Sigmonster 300-800mm is a good choice,. but if cost is no object and long distance is worth more than zoom flexibilyt the Canon EF 600mm f/4L IS is aguably a better solution.
Sigma offers 800mm @ f/5.6
The 600mm with a 1.4X T-con offers 840mm @ f/5.6 and does this with the very significant addition of Image Stabilization. Birders use the 600mm with the 1.4X attached all day.. and image quality remains superb even wide open.
At these focal lengths the increase in keepers that IS offers is exponential...
Also,. if using a 1 series,. the 600mm will AF with the 2X and results are still pretty good when stopped down to say f/11
Either of these lenses though are in a world of there own size and weight wise,. say 13 pounds?
The lighter weight solution,. at about the same price as a Sigmonster is the Canon 500mm f/4L IS.
Used with a 1.4X you get 700mm f/5.6 and IS. On a 1 series a useable 1000mm f/8 IS (better when stopped down to f/11 at least ) Much lighter than the other two.