There's no base level or other identifying information on this attribute (we call it a "spell," but really, given its information in the game, it seems like a gear-specific ability), so we can assume that it'll be a full 1-80 bonus, just like the current heirloom gear in the game. Unfortunately, we haven't actually uncovered any new heirloom items yet, so we have no idea if this stuff will show up in patch 3.3 or the expansion. And of course, just because the bonus applies from 1-80 doesn't mean the gear can be worn that whole time -- it sure seems like Blizzard wouldn't necessarily want Worgen and Goblin alts speeding past Cataclysm lowbie content. But we'll see -- in the meantime, look for a little extra heirloom power, coming soon.The sneaky database over at MMO-Champion has uncovered a little spell/aura/attribute hidden in the patch 3.3 PTR files that hints at some extra XP from future versions of heirloom gear. Currently, with two pieces of heirloom gear, you can get up to a 20% bonus (the chest and shoulders each give a stackable 10%, while heirloom weapons give nothing). But this Heirloom Experience Bonus +5% "spell" grants an extra 5% experience, so it could be that in the future, we'll see pieces of heirloom gear that have an extra bit of bonus on them -- maybe gloves, cloak, or a belt -- that will bring the total bonus XP up to 25%.