Andy I tried to answer your post a little earlier, but the link I included as again caused the post to go for moderation.
Briefly the 550D does not allow you to reduce the flash power and it's doing this that increases the flash speed. I'm not sure if you followed the link I provided in the Comp thread and unfortunately this site will not accept it, but if you go to
http://www.cameratechnica.com/ scroll down to High Speed Photography With Your Flash, this will explain it all.
Harry