Benn at 160lbs had frightening power and was fast and exceedingly aggressive. As such he's bound to catch Floyd early with something that'll be too much for a former jnr lightweight to handle...especially as Floyd would be slower and less mobile at this weight.
mayweather jr is not a very good welterweight in the grand scheme of things. yet the snooty canuck da'suction of gaymen thinks he can compete with middleweights.
Well Floyd moved up 24 pounds to face De La Hoya...but I guess you're right in that there doesn't exist a precedent in Floyd's career for this type of move....