I think they should remove the XP bonus from all heirlooms.
Instead there should be an Achievement for getting characters to max level. then in each capital there should be an NPC that sells a potion or potions with varied levels of XP bonus for a small amount of gold that can only be bought if you have the Achievements.
So getting one toon to 110 you earn "Elder of Azeroth" which lets you buy a +10% Flask of XP for 24 hours.
getting a second toon to max level gets you "Elder of Azeroth II" which lets you buy +20 and so on.
this way players with max level toons and opt to BUY the potions they need for a small amount of gold to speed along their process of leveling, or they can just level as normal, this would allow people to use heirlooms but still level at the same pace as new players who dont have any.
Options....