My first Pazuzu kill (Or I should say leech) was at level 5, and I gained roughly 2k Xp. for the same playtime at lvl 4, i gained around 10k by killing Harpeia and Teles. So yes, the level difference makes leeching high level trains really not worhtwhile, unless you just love being able to AFK.
Besides, while popping low level FATEs, you're also dropping a decent amount of crystals, on top of what you gain from the FATEs, while I've never dropped any crystal when my party killed monsters that ware far higher than my level.