I bought a 200-600 yesterday morning to photograph a competitive cricket match, I was not happy with the sharpness when I checked at 100% - but perhaps it was atmospheric disturbance, so this morning I did some 'proper' tests. Shot from the same position, cropped to match (600mm has advantage in cropping)
I discovered the 200-600 @ 600mm f6.3 is not as sharp as the 100-400 f5.6 even after it's been cropped in to match - which I assume means I could have shot the cricket on my 100-400 and go sharper images even if I needed to crop.
near = about 10 feet
far = about 20 feet
A1108456 by
Daniel Cook, on Flickr
A1108454 by
Daniel Cook, on Flickr
A1108458 by
Daniel Cook, on Flickr
A1108459 by
Daniel Cook, on Flickr
with these kind of results i'd be better off shooting 100-400 and cropping.