Regarding the flash, sometimes it is on and sometimes not--I have been experimenting with this. I have shot football at night without it but am of the opinion there has been some help on shorter range shots. Also I beleive I get better results if I shoot at speeds greater than 1/250--which is why I sometimes use hi speed sync on the flash. By the way all the football games I cover are at night and some high school field lighting is better than others.
High-Speed Sync is a special mode and is not really designed for high-speed photography - For example @ the same 1/250s shutter speed the regular GN is 55, but is cut to 27.5 when in HSS mode (i.e the output power is cut in 1/2) and is reduced lower as the shutter speed increase (check page 121 of your 550EX manual for other shutter speeds)... so the maximum distance is not contant but will vary with the shutter speed used as the flash fire smaller and smaller strobocospic burst to keep up with the narrower and narrower traveling shutter slit:
I'm also all set for using fill-flash however when used in artificial light (instead of daylight), you have two different temperature colors fighting for one another so be careful - Notice the yellow cast? The 550EX will fight back with blue instead... which will be even more apparent with indoor ceiling lights