I've been doing this to lvl countless of 90s with friends, I did all the runs so I'll share all the info I know here instead of making a new thread.
I don't know how the math works exactly but instead of adding the % from the heirlooms it looks like it multiplies it, which is why the amount of exp for the completion of the dungeon is so high.
The best route for Horde is:
10 - RFC
23 - SM
37 - Stratholme
48 - LBRS (remember to loot the Roughshood Pike on the wall and Highlord Omokk's head in order to spawn Urok)
58 - Hellfire Ramparts
63 - Mana Tombs
70 - Mana Tombs Heroic, Sethekk Halls Heroic
72 - Azjol-Nerub
75 - Halls of Lighting
80 - Stonecore
Note: I did these runs with as a 90 mage for various reasons: portals, double blink, sprint.
Note 2: For alliance you could just replace RFC with Deadmines.