Originally posted by dminstrel For instance keep my Tamron 70-300 and get good results with f/5.6 and 1/1000 at ISO 12800+?
You can get an extra stop of light (so ISO 6400) by setting your shutter to 1/500. Actually, you could go to 1/250 and ISO 3200, no one runs that fast with a ball and frankly, pictures of players running by themselves without the ball aren't that interesting and if their feet are blurry, but their jerseys are sharp, no one will complain. The problem is timing your shot and getting the good parts in focus. To keep a player in focus you need to pan with the player, which is another reason why you don't need such fast shutter speeds. F5.6 isn't a bad thing, either. The deeper DOF compared to a faster lens means that getting the right parts in focus isn't as critical.
Photo below was taken at 1/160 sec. with a manual focus 70-210mm lens at ISO 8000 on my K-30.