Well that's a bit mis leading and a bit wrong.
I should post the specifics per camera model but don’t have time to go get the pdf manual for each body and
turn to the AF section and read the insert about 2.8 lenses.
Read this yourself in the manual for a 1D MKIII and a 5D and a 7D and a 40D and a Rebel.
Each Body have different specs on how they react to a f/2.8 and f/5.6 and f/8 lenses
and on some of the bodies it make almost no diff at all.
Use a real F/4 lens and use a real F/2.8 lens to compare AF!
The TC's introduce a whole set of AF problems completely unrelated to f stop that will show a slowdown and accuracy loss of AF.
The full Accuracy gain of a 2.8 lens isn’t even really realized until on a 1D body anyhow.
The non 1D body’s usually only increase AF on a 2.8 lens for the Center AF point only.
The 1D body’s have increased AF accuracy on many many more AF points.
The new 7D has increased AF accuracy in middle between a 5D and 1D since it has a few more triple sensitive AF points like the 1D has, but not as many.
-Steve
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