I think it only boosts the mob xp drop, not the xp amount in an xp orb in general, otherwise you could generate XP by cycling it through the player.
Considering there are so many preventions against exploits (which I love about Reika's mods ) I guess it makes sense. Well back to the mob-spawn idea. Still fighting with the idea of using diamond-dollies for getting spawners. They're so tempting but so easy to use