IS is a very useful tool, however it's not magic. I'd shoot the focal lengh for your shutter speed when youre streached out to 400. You can get away with one stop less, but you better be on a tripod or a support of another kind. It's real hard to sharpen out of focus shots. Youre better off to up the ISO, shoot RAW and under exposing the shots by a stop or two then bring back the exposure in PS.