Upper hand? All they will have over a leveled character is a 483 weapon to start with, and possibly 1-2 trinkets. Timeless isle kind of negates your leveling gear as you pretty much hit 480+ ilvl within 30mins on the isle. With the upgrade to the weapons aswell and how cheap they are really no major bonus to the gear.
On the other side, that boosted person is missing out on thousands and thousands of gold made while leveling. If its a new player, they wont have flying/epic flying and all forms etc until they farm the gold up for it. My last toon, even with spending 3k on proffessions made me a further 3k after training etc by 90. 6-10k isnt uncommon so thats a big loss in gold.