There was a post somewhere on the official forums that stated the CoF farm is not an exploit, because the mobs being killed only reward xp. No loot or anything else.

That said, I already have pact mastery done and I've started working on precursor crafting as of last night. XP boosters (specifically kill streak boosters) are your friend. I did a mix of Harathi Hinterlands (where you kill wave after wave of centaurs) for one of the daily event completer achieves the other day and was seeing ~400k xp per event. Not to mention a truckload of loot. Cursed Shore also works well if you get in on a map with decent amount of people (although old timers may cringe at the idea of going back to CS yet again).