Flash for sports? Not a good idea.
The 580 EX ii has a guide number of 58 meters or 191 feet. Flash exposure is independent, essentially, from shutter speed and is calculated by: Distance = Guide Number/Aperture. 191 ft / f/2.8 = Flash distance of 68 feet at ISO 100. At ISO 400, the guide number is multiplied by 2. So flash could theoretically help out to 136 feet. Do you want to spray an athletic team, in the midst of competition, with a full power flash blast? Will the venue/coaches even consider letting you do that? Methinks, not! Especially at night! During the day you may get away with it, but you won't NEED it. And it won't help.
At night it is even worse. If you use normal flash, you are limited to normal sync-speed of 1/200 or 1/250. That is just barely fast enough to get good shots with a 200mm lens on a crop body, for most of us. Compound that with the need of higher shutter speeds for sports, and you are now in the FP flash area, which cuts down on the reach of your flash. A full-power flash at ISO 400 and 1/1000 is good for about 30 feet. Add all of this up and you get why the professional shell out big-$$$$ for really fast glass, and sturdy mono-pods. You just can't get it done with a hot-shoe mounted flash unit. There is not enough power.
In bright sunshine you can shoot ISO 100-400 with an f/2.8 lens and keep the shutter speed faster than 1/1000. Focal length does not matter here. You can do this with a 24mm f/2.8 or a 400mm f/2.8L. That is as good as it gets. The laws of physics will not be defied.
Day or night, flash doesn't even enter into the equation. Flash does not do much for sports, without getting in the way of the competition. Do the competitors a favor. Leave it at home.