You could always look at the 70-200mm f/2.8 L (non is version). You are going to be shooting at a high shutter speed anyway so IS isn't going to do much good if your just using for dog agility. In addition you get the high precision points of having a 2.8 lens. I believe that is priced right in that range also. I personally use my 70-200mm f/4 L IS, 85mm f1.8, and my 100mm macro USM If you limit the focus on the macro it is just as quick as the others. There is also the 100mm f/2 which is the twin of the 85mm. Lots of people love that lens.