There is no doubt that Instance boosting is faster than questing if you have the right class and gear for the job. With slight slack I got my group of Shamans to level 40 in roughly a day /played powerleveling them in SM most of the time.
I'd say try it out, see what kind of exp per hour you get and compare it to the leveling guides, though I don't know if they've been updated after the leveling nerf/buff.