I've been using a Tamron 200-500mm zoom along with a Tamron 1.4X teleconverter on my 50D.  As expected the autofocus is hit or miss, even in good light. I had been using only the center focus point since I didn't want the autofiocus to pick up unwanted objects in the foreground.  By accident I selected one of the other focus points and found the autofocus success rate improved quite a bit.  I know the center focus point is different than the others and provides higher accuracy with f2.8 or better lenses but it also seems to require somewhat more light than the others to focus with slow lenses.
I then tried turning on all the focus points and aiming the camera at a wall so all of the focus sensors  were seeing essentially the same image. In all cases one or more of the outer focus points would lock on the target  but never the center one.  I think the camera is OK since it focuses properly on all sensors with "legal" aperature lenses. Has anyone else found this to be the case?