I've been shooting BIF handheld for quite a few years. Recently I am using a new telephoto and have mostly shot in IS mode 3. I am wondering if there is a high shutter speed at which sharpness begins to suffer if IS is on? For instance, shooting at shutter speed 1/2000 with 400mm handheld would IS be more likely to help or hurt sharpness? Anyone know the frequency response of Canon IS on their newer telephoto lenses?