I have the Sigma 500mm and have used the 1.4 extender with it and was not terribly happy with the image degradation. That said, I love my lens capability at just base focal length and at 800mm, you should be in good shape, for the money you might want to also consider the Nikon 200-400mm with the Nikon 2.0 extender, I know photographer's who use that setup for western big game who take absolutely phenomenal photo's.