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They are pretty much the same AF system, but the T2i may seem to have a slight tracking advantage as its a lower FPS so has longer between frames to track the target. Back when I had my 40D (a slight advancement in AF system) I found the only way to get any sort of tracking performance was with the fps turned down to slow (3 fps) . 2 or 3 in focus shots per second is better than 6 OOF ones
As for file size, well I've not seen the circuit layout or where the crowd will be allowed to view from but I bet you won't get that close. Ok this example is not F1 (bikes are smaller) but it gives an example.
And here is the file I cropped it from
That was shot at silverstone last from the front row of a stand . I used the 100-400 on a 7D . I shot a few thousand images from many different vantage points around the circuit and was normally as close as I could get at each (mostly from the seating as I was with my dad , but the seating there nowadays is as close as you get most of the time with that advance of being above the fences) . I would say that in most or all of the places I was reach limited and have had to crop down to about 8mp .
I shot a car event from there a couple of months later (WEC) and I still had to crop a lot of the shots .
Oh one last thing, I found te 100-400 worked best if pre focused on the apex of the corner and shot in small bursts . I did shoot a few head on shots at the car event but found that even a quite slow section the 7D 100-400 combo had quite a few OOF ones in the set (still had some good ones though)