Argument for Nikon 200-500 against Tamron/Sigma

One thing I have noticed is missing from the arguments over whether the Nikon will be better than the Tamron and Sigma 150-600 lenses is autofocus points and their relation to the maximum lens aperture. The Tamron and Sigma maximum lens aperture from somewhere in the middle of the zoom range until all the way at 600mm is f/6.3. The Nikon is f/5.6 all the way to 500mm. What implications does this have?

Well, for a high performance autofocus system like the one on the D800/D800E/D4s, using the Nikon 200-500, you have 15 cross-type AF sensors in the middle and use of the remaining 36 line-type sensors in the array of 51 sensors.

When your maximum lens aperture is smaller than f/5.6 but f/8 or larger (i.e. for the Sigma and Tamron), you only get nine cross-type AF sensors in the very center, and 3 line-type sensors on either side.

I mean, it could be worse. If you use an f/4 lens with a 2x teleconverter (f/8 maximum), you only get one cross-type sensor in the center with one line-type sensor above and below and four line-type sensors on either side.

Does the D800/D800E/D4S allow you to use all the sensors at 600mm with the Tamron and Sigma? I don't know, but it would result in unreliable autofocus performance.

And then think of cameras with lesser autofocus systems...

NOTES:

Autofocus system details: http://nps.nikonimaging.com/technical_solutions/d4_tips/autofocus/

Tamron 150-600: 427-600mm is f/6.3: http://www.the-digital-picture.com/Reviews/Tamron-150-600mm-f-5-6.3-Di-VC-USD-Lens.aspx
Sigma 150-600 Sports: 321mm-600mm is f/6.3 http://www.the-digital-picture.com/Reviews/Sigma-150-600mm-f-5-6.3-DG-OS-HSM-Sports-Lens.aspx
Sigma 150-600 Contemporary: 388mm-600mm is f/6.3 http://www.the-digital-picture.com/Reviews/Sigma-150-600mm-f-5-6.3-DG-OS-HSM-Contemporary-Lens.aspx