There's a HUGE penalty to XP once you hit 85 if you aren't in Pandaria. The quests themselves still give the same XP as before but actual mob xp gets severely reduced.
This is mainly due to the fact that a level 85/84 Pandaria mob on the world map has significantly more HP than a level 85 cata world mob.
Riight. Time to move on then. Thanks for the tip.