I would believe leveling using multiboxing would even be more easy than single char leveling in endgame cataclysm content (75-85).

They changed mobs to have a lot of life and to hit like a truck. So if you level 2 characters instead of one, every kind of kill quest just will be done much faster.

I only got one single account on beta, and currently leveling one of my chars in uldum. You need to use cooldowns if you pull 2 mobs, and it takes up to a minute to kill both. And they hit for 3-5k noncrit and 8-15k crit.

If you do that as a 5 toon team, it should be really easy. The collection quests are a pain in the ass, yes, but there are not many quests which need to collect a lot of items.