claims to level 92-98 in 1.5 hours in heirlooms back in 6.1
from the recent comments :" It has been slightly nerfed so after 95-96 won't be more efficient than questing".
remember that normal leveling up to the outposts quest lines of each zone (that rewards the books) gives you thousands of gold and a lot of garrison resources to start.
So I am not sure if it is worth it just to save some hours.