#1. "RE: What does this mean? Sensor works all the way down to f/8" In response to Reply # 0
They aren't talking about the aperture you are shooting at. The aperture in question is the wide-open aperture of the lens you have mounted on the camera. The D7000 AF is rated to work with lenses that are f/5.6 or wider. When you put a teleconverter on a lens, the wide-open aperture is reduced, in some cases below f/5.6. It is for those cases that the more sensitive sensor is designed.
However, the sensor is more sensitive with any lens, thus it works better in low light. (I have both the D7000 and the D7100. The D7100 AF system is distinctly superior, and not just because of the f/8 AF point.)
#2. "RE: What does this mean? Sensor works all the way down to f/8" In response to Reply # 1
Thanks for the reply Jon.
If I understand you correctly, when the camera is focusing the lens aperture is always wide open and does not stop down to your selected aperture until you actually open the shutter. So if I had a lens with a max aperture of f/5.6 and had my aperture set to f/16, the camera would focus at f/5.6 wide open and step down to f/16 after it has focused and I release the shutter.
If this is the case the statement would make sense to me, as they are basically saying it will still focus with the low light let into the camera using a really slow lens that can only open up to a widest aperture of f/8, or a lens with a max aperture of f/4 when using a 2x teleconverter effectively making it have a max aperture of f/8.