The front sway bar in the F is the same as the F-Sport sway bar. As has been stated the rear sway bar is 17mm in the F and 19mm for the F-Sport Kit. The shape is also different. The F-Sport is straight so it's shorter. I have the F-Sport rear, and it's a worthwhile mod. More neutral handling and flatter cornering.
The IS Sway Bar Specs are attached.
Lou