Tails wrote:
He has come to the conclusion which I stated earlier, that the fastest way to level, is to do dungeon quests. After that, it is questing in level appropriate zones. Sure, he's selling a product and all, but you don't have to buy his guides to take advantage of the faster leveling speeds. So, that would be my "evidence."
Jame's guide (which is free) also mentions that the highest xp/hour is achieved by doing quests in a certain order (which is what his guide is all about), not by grinding away on mobs above your own level.
Mobs five levels above you have increased mitigation and health, and they hit harder, increasing your downtime. This is amplified greatly if you grind expansion content from a level below that expansion (e.g. grinding mobs in Northrend at level 65).
I'd say the current way to achieve maximum xp/hour would be to run quests while queued up for dungeons. The dungeon bonus coupled with the high kills/hour and the efficiency of doing quests while grinding mobs (most quests require you to grind mobs) would undoubtedly provide more experience per hour than just killing stuff that takes longer to kill. That's been my personal experience throughout the years, anyway, as I always try the grind route when a new expansion is released and it hasn't paid off yet.
Now, grinding mobs above your own level obviously have some benefits, such as the items they drop which can usually be sold for more gold than the items from the previous expansion, and can provide you with some early upgrades. Those early upgrades, however, would increase your xp/hour by a lot more if you went back to doing level appropriate content as you'd effectively "out-gear" it.
In the end, though, it's all about preference. Some don't mind grinding away on stuff, some don't mind it if they gain something from it and others, like me, are tired of mindless grinding and want a more efficient way to the level cap.
Play the game the way you want to play it.