considering the 70-300 vs 70-200 f/2.8 + 1.4x extenter, I really wouldn't know which is better AF-wise.I can try to answer that for the 60D: Since the 70-200/2.8is2 is not the latest generation of Canon lenses - time moves fast - the af speed is lowered when using an extender. Other than the said 1D, on the 60D all 9 af points are cross up to f5.6 (except for a few legacy lenses, see link below). Thus, the faster af speed of the 70-300 should outweigh the difference to a slower f4 because the "high precision" center af is only available at f2.8 and better.
I use the 70-300L on the 1Ds3 for head and shoulders personal portraits where I find the clients prefer f/4 or f/5.6.That's my personal experience, too - the reason is that a too bokehish background looks like the subject was standing in front of a green screen when shooting and then the background was replaced with a blur. Maybe this somehow seems to focus the eye too much on the portrait and the person's "shortcomings"? Or is it because you cannot get the tip of the nose into the dof at the same time as the ears with 2.8?