Hello!
I found this quite interesting post on wowinsider.
Apparently Hellscream, the final boss in the new raid coming in 5.4 will have a chance to drop a heirloom that is for level 90-100 only. (This might change, it's from the PTR, not evrything here will come live) I think this pretty much confirms that 5.4 will be the last patch in Mist of Pandaria since, why else release a heirloom if you have to wait another patch to use it? It also confirms that the level cap in the next xpac will be 100 (Unless ofcourse they want to keep the heirloom to be used in the xpac after that, but i doubt it.). If you want to read more, check the link to wowinsider HERE.