Floyd doesn't use a shell. Winky Wright and Clottey have used the shell. Hands high, gloves covering face, arms in tight so you don't have much to hit except for the forearms.

Mayweather uses the shoulder roll and anticipation which isn't a shell defense. He looks to counter shots while the "shell guys" like Wright and Clottey just stand there and try to block and when you finish throwing they try to throw back.

As for RJJ's speed during his prime vs Floyd's shoulder roll, I think Floyd's defense is better. While Floyd did avoid a few fighters, he faced better competition as a whole than Roy did during his prime.