I have a question, a serious question that could influence my personal financial statements for awhile if not executed well... I already have the Canon f/4.5-5.6 100-400L IS -- and I'll note that this lens is one of favorite and most useful lenses... but in low light even at f/4.5 and ISO 3200 on my 5D, I can't get fast enough shots in a place I like to photograph that prefers no flashes. It's a commodities exchange, open-outcry environment, and traders are frantically yelling at the opening and closing of the market, shoulder to shoulder. It's a total crapshoot as to whether or not my shots of the traders will work or not. You think the f/2.8 70-200L IS will help much in getting faster shutter speeds? Trying to get photos of the traders in their huddle, yelling and berating each other at the open and close. I'd be interested to hear from those with experience with both the 100-400 and the 70-200 f/2.8. Trying to prepare for dropping 2 grand on a friggin' lens.