in answer to the "why" f8 is a limiting aperture, i think i found a satisfactory answer on canon's website
it gets into some basic concepts of how those AF sensors work - pretty interesting actually
basically, the wider the aperture, the more precise your focus - alternatively, the smaller the aperture, the less precision possible
basically, they had to make a design decision - do they tolerate softer images, or stick to their guns and ensure higher quality photos?
these paragraphs are from page 2...
Where does the term "high precision" come into play? The farther apart the two rows of pixels are, the more finely the system can adjust the focus, and the more accuracy it can apply to determining whether focus is "close" or "dead-on". Slight deviations in where the twin beams are focused, if they are off-center, now can be measured in finer increments. Canon EOS SLRs with high-precision AF sensors simply move the pairs of sensors much farther apart, and accordingly, the AF information can be more finely broken-down and reacted to. So why not simply move all the line sensors farther apart? There's a limit, and that is the widest aperture of the lenses a shooter might use. To adequately cover the entire AF sensor array, every AF sensor requires lenses (or a lens + tele extender combination) that's a certain f/stop or wider in order to assure proper AF operation. Standard-precision AF sensors, positioned relatively closely, can get by with lenses f/5.6 or faster. This is an advantage in some ways, because nearly all AF lenses can be used without limitations.http://learn.usa.canon.com/resources/articles/2011/1dx_af_precision_crosstype_article.shtml
But high-precision AF sensors have traditionally required lenses with f/2.8 or faster apertures, since each high-precision AF line pair is farther apart, and requires a physically larger "cone of light" coming into the AF system to fully cover the rows of pixels. When a lens slower than f/2.8 is used, those line pairs no longer function, and (usually) the camera reverts to another pair of standard-precision AF sensors. But high-precision AF sensors provide from 2x to 3x the AF accuracy at the sensor of more traditional designs, and that accuracy kicks-in when a wide-aperture lens is mounted. In other words, high-precision becomes available when it's needed most – when lenses with very narrow depth-of-field are mounted and being focused.
very interesting article - you should give it a read - they explain it quite well
before this thread though, i had no idea how AF worked