Or as soon as the first raid opens.
I just finished doing this on the beta moments ago. I did two quests that showed 22050xp in the quest log and 24255xp when turning in (and accepting the quest) due to the 10% from Fast Track. I then turned in one without any heirlooms and one with the shoulders equipped. Both gave the exact same xp (24255), so heirloom at 80 is useless as far as bonus xp is concerned.