While I share your affection for Rugby...American football players would die in droves if they took all the "armor" off given the speed and size of the players.
And I'm not just talking about the professional or college game. Even the high school teams have some HUGE, FAST kids! I know I didn't play against 300-350 pound kids with 40 yard dash speeds between 4.5 and 4.8 second back when I was in in High School....geezus.
Cobblers

There are equally massive Rugby players who dont need to strap on a plastic tank under their shirts to stop them getting a little bruise.
Bill Cavabuti was around the 370lbs range and ran like the wind, Jonah Lohmu was not far behind that, Dallaglio, Uini Antonio, all massive fast men, all unarmoured but for big squishy ears and a gum shield
