How much difference does it make to use Panning IS versus NO IS? I can see the Canon 70-200 f/2.8L IS in the near future (maybe in the next month if I sell some unused gear). Auto racing season is beginning to start. Ability to pan is major. I am at the point, where (with my Sigma) ambient lighting is at 1/60, f/2.8, 1600ISO. However, at 1/60, my panning technique only allows for 20% of the photos to be clear enough. Bumped up to 1/80th, the images are a bit dark, but I get an 80% keeper rate. So, if PANNING IS will help get more of those 1/60th panning shots, that would be super!