This is all theory, I'll say that right now. I cannot prove this info will actually be useful, or that it will work, but there are a few items, most added in Warlords, that could in theory help get a head start of the competition while leveling through Legion.
The first of these items is one that has been available for a while and in theory shouldn't be any different for Legion:
Battle Standard of Coordination among its other varying levelsof Unity and of Cooperation
They used to and I believe still do share a cooldown, so you only need to pick up one of them. This item boots EXP gains by 15% for all kills done within 100 yards of where it is placed. Note that if you leave that 100 yards you will love the buff and be unable to get it back for the duration of it CD
These next few items are all treasures found throughout Warlords, most of them in the Spires of Arak and one in Frostfire Ridge, but they all do the same thing which is they give 1 bar of rested experience per item used. Now that does not mean they give 1 bar of experience, it means that each item upon usage gives you one bar of rested experience, which increases EXP gains by 200% for its duration. The items are:
Ravenmother Offering #1
Ravenmother Offering #2
Ravenmother Offering #3
Ravenmother Offering #4
Ravenmother Offering #5
Obsidian Forstwolf Petroglyph
Those are items which may work and hopefully you have not pocked up and used yet. This next item is one everyone probably got and unless you were very forward thinking it is no longer available to you.
Celebration Package from WoW's 11th Anniversary, which grants 11% increased Exp and Rep. Now this item was only available from the package delivered when you log in during WoW's Anniversary, and once that event was over, the item was removed from your inventory. These items actually are only removed once they are taken out from the package, so if you were very forward thinking this is available to you still in the wrapping, and it will only be removed once you log out or are DC'd, since that's what Blizz checks your inventory
Feel free to comment anything that I may have missed, and comment if I got anything incorrect or that has changed.
Good luck hunting these items down and Happy leveling faster than those around you lol