I would pass on this job. This would be a tough job with my 200mmf2.8 lens which is 2 stops faster than your lens. You will not have enough light to get a fast enough shutter speed to stop the action and the rules for Dog Agility does NOT allow the use of flash for safety concerns of the dog. If you raise the ISO high enough to get the shutter speed you need to stop the action, the mat and the black dogs will show significant digital noise and you won't be happy with the photos. I doubt that Noise Ninja would be able to remove the amount of noise you will encounter. Sorry to have such a negative post, but I have been there, done that.