For AF accuracy, smaller steps in the stepper motor is all that's needed. Which probably explains why those very fast aperture(f/1.4-f/1.8) type lenses don't focus as fast as they otherwise could.
This doesn't imply that fast f/1.4 lenses don't focus quickly tho ... it's just that in all probability, the use of smaller steps for the stepper motors means that more steps are required to be accounted for as the lens focuses from near to far and back again.[./quote]
It shouldn't be too hard to fool even an AF-S type lens into communicating a smaller aperture to the camera.
A very thin extension tube with the right electronics between camera and lens will easily do the trick!
The issue will of course be the loss of infinity focus, but if the extension was small enough this may not necessarily happen to all lenses.
Program the CPU engine in the thin extension tube to communicate to the camera that the use of a 2x converter is present and the f/2.8 lens becomes an apparent f/5.6 lens.
That I know .. no such extension tube exists .. and an AF-S capable 2x TC would need to be cannibalized to produce such trickery.
The alternative doesn't even bear thinking about!
Replace the CPU in the very fast (say f/2.8 or f/1.4 lens) with one that tells the camera is much slower.
Who's willing to potentially destroy a multi thousand dollar lens for the sake of greater knowledge