One of the main complaints about the 5DMKII is it's focusing system, so I doubt you will come close to the D7000 with Tokina 10-17, no matter which lens you use. The type of action you describe of a swimmer moving through water requires a focusing system that tracks well, as the D7000 does. Very different from a moving baby that you can pretty much aim the central focus point on.
That's not fully accurate, Jeff and too Rockwellian a statement
! The 5D2's AF is actually faster than the D7000 in acquiring focus on subject, but the shutter lag is a bit slower, so it's similar in good light. Where it does start to lag
is in low light/contrast conditions, where subject acquisition becomes a lot slower and less reliable. That's why the 5D3 , while not the big jump the D800 is, is what the 5D2 should have been in 2008.
The 5D2 with the 8-15 will focus as fast if not faster than D7000, BUT at 3x the cost of a D7000/10-17 combo.
It is true that if you don't fill the AF frame, the D7000 39 pt system will probably track better, since it's better spread across the VF. However some of the unhappiness of people about the 5D2 AF system comes from not understanding that it had smaller AF sensor area than APS-C because the viewfinder was BIGGER!
Pauline, is there any reason why you'd want a FF camera if you already have the D7000? The 5D2 has slower fps and with the 15mm FE an older AF system.
I would also suggest you use the AF-On button to start AF on the 5D2. It allows better control of focus.