my 150-600mm sport seems to focus pretty quick.. its heavy though.. nice IQ and sharpness, mine focused perfectly out of the box some people seem to need to use the sigma dock to dial it in. great lens. from what i see of the niko 200-500mm IQ is also very good, it weighs less ..
I have to admit that i really like the sport from everything i've read and heard. I put sigma everything on my cam, they've been coming out with nothing but great stuff. But their lenses seem to always pack way more glass. I remember when i replaced my 50 1.4g with the 50 1.4 art, it was so much heavier. The sigma sport for example has 24 elements in 16 groups, while the nikon has 19 elements in 12 group. But i'll see and test both the nikon and sigma when they get here.