Currently to get from level 7-15 you need a million xp (just using a number). If you were to use all the xp buffs before using the stone, it might put you at 1.5 mill xp, and that would mean that you would be level 16 almost 18.
In the future when Turbine adjusts the amount of xp needed for a tr, the amount of xp may be shifted away from the back end (18-20) and they may spread the xp more to the middle levels. So we will just say in the future it takes 1.5 mill xp to get to from 7-15. If you multiply 1.5 mill times all your xp buffs, you may be lvl 18 and almost to level 19.
This is just made up numbers, but if the buffs were to stack with an Otto’s box (under the new xp table that they may make), you would have toons taking the stone and ending up at level 22 (if they spread the xp around evenly at mid-level and not just the back end).
This is just speculation, but I thought I saw that they may be readjusting the xp. I wish Turbine would have been up front that they were “fixing” this, but I can understand why they may be doing it.