No, sorry, but I strongly and respectfully disagree. Mode 2 only stabilizes in the horizontal direction, which is going to create some odd focusing issues if you are moving your rig in all directions.
Secondly, and more appropriate, IS is intended to correct for camera shake and movement, not subject movement, and the shutter speeds you should be using for sports far exceed those where IS would have a positive impact (1/(focal length), anyway.
It will noticeably slow down AF, and if you use it on a tripod or monopod, the sensing system internal to the lens takes up to a second of holding down the AF button to recognize that the lens is mounted to a 'pod every time you shoot, and who holds down their AF button for a second before taking a sports shot?
Plus, do you really want little gyros and motors moving your glass around inside your lens while you're swinging your lens around, trying to catch a 95 mph fastball, or a running back diving for a ball at 1/6400s, or a hockey puck flying into a goalie's glove captured at 1/2000s? I know I don't.
Whenever I have slow focusing issues with my 400 IS, I can guarantee that somehow the IS has gotten turned on, and this has become the first place for me to look to correct an AF issue.
IS is primarily for low light images where the subject is relatively stationary (wildlife, for one) and Mode 2's only action/sports application is for horizontal panning as in motorsports.
But, don't believe me. Here is the definitive source:
http://web.canon.jp/imaging/lens/index.html
2. I don't love the results I've seen with flash.
I understand, when done poorly, flash images can look odd. However, if you learn to use it correctly, I can guarantee you that your shots will look MUCH better than anything you can get shooting ambient - and this holds true even with a MkIII. You just can't get adequate color saturation, proper face exposure, etc., or even white balance, shooting ambient.
With flash, I can stop action at 1/800s (flash duration at full power), get light under helmets to expose the faces properly, get colors that pop, whites that are white (and not brown or yellow), etc.
But, it's not easy to learn how to shoot night football with flash, especially compared to just going out to the field, dialing up the ISO, opening your lens all the way, and firing away.