I agree with most everything JohnG
In this mode the camera focuses 5 times. That takes time and sometimes the camera has more trouble than others predicting and focusing so your frame rate will go down. The key is to make sure youo focus and track before firing, because the first shot is shutter-priority
What was neglected is that the camera only allows certain amount of time-out per focus: 200us for USM/HSM and 250us for non-USM lens which translate to 5 or 4 shots/seconds...(there's a link somewhere). Again do the lens cap test. The camera will keep up the frame rate even if nothing is in focus - Just check it everyone and convince yourself of this shutter-priority mode
Now the MrkII can do 8fps - and here I agree with JohnG
that in one-shot AF the camera can achieve that rate, but it will fall down to 5fps if
the lens can't achieve focus (it will try but the time-out will overide it) - again check it with the lens cap on... especially with non-USM lens (i.e. 4 vs 8fps)!
The bottom line is this:
o With one-shot AF at least the first frame will be in focus (the shutter won't releas otherwise) - and yes you can do a burst as long as the subject stays in the DOF field and you'll will be kosher
o In servo-AF, the first frame is not guaranteed to be in focus to maintain the shutter-priority, but the camera will try to AF between shot to track the subject if it can (i.e. under the time-out period) otherwise the camera will try to maintain the frame rate even if it can't focus!
... Yes I dabble with Sport once in a while (again in one-shot AF