Can't really answer for either you or a Bigma but here's a shot taken with the Sigma 500mm F/4.5 and Kenko 1.4x TC (taped to allow AF). This shot taken in the garden (out the open window!) - I've others from further afield but I don't display them on the internet!
Canon 7D, Sigma 500mm F/4.5, Kenko Pro300 1.4x TC, tripod