I'm giving somewhat serious consideration to a 5D3. Because I shoot sports, my constant struggle is to get enough light to keep the shutter speeds as high as I need them. So I've always looked for the fastest lenses I can get, with f/2.8 being the minimum. Of course with a fast moving target, when the DOF gets too thin, even the slightest miss on the AF starts to show, so it's a bit of a two-edged sword.
So the question is, with the great ISO available on the 5D3, will I start to be better friends with my 24-105L and can I consider the 17-40L (both of which are f/4.0) or should I still stick with the faster lenses?
If you're going to shoot sports with the 5D Mark III, I'd consider a few things. For basketball, I'm using a 135L and a 200L, with a backup 85 f/1.8 for sideline/under the basket shots. All are f/2 or wider. I don't shoot anywhere near f/2 though. These lenses just happen to have the ability to focus quickly. I stop them down quite a bit actually, set my shutter speed to 1/500, and ISO to about 3200 or 6400 on a 5D III I'd guess. I try to shoot around f/4 to f/6.3 in indoor lighting, with sufficient ISO to maintain 1/500s shutter speed. Preferably outdoor sports it's easy to shoot 1/800 or 1/1000, which is better. Since you won't likely shoot wider than f/4, the actual aperture is not the issue, it's the focusing speed of the lens itself, as well as focusing accuracy. This is where the 135L, 200L, 300L, 400L all accel. Obviously you don't need the 300's and 400's for indoor sports, but for football you would, and also track and field.
Given all of that, higher ISO settings on the 5D Mark III will actually outperform my 1D Mark IV in comparison tests in similar lighting. For instance, my 5D III's outperform my 1D4 at ISO 6400 by quite a bit it's looking like, so your camera should be great.
