Quests : It's gonna be a living hell, can we work that out ?

[Apatheist]

almost half of 'questing' XP comes from kills

True but I was also pretty generous with the numbers. Better to underestimate than exaggerate. It's pretty unlikely anybody will be able to find 10 kill quests in a single area that aren't gated behind collection chains and then complete them all in 20 minutes each. Particularly on the open world map with people competing for spawns, being ganked, travel time, etc.

I was also pretty conservative with my grinding numbers. 4 Mobs at 168XP per pull and around 1 minute to kill and rest would be around 141,000XP. A bit less than double the quest value in the same 3.5 hour period. An efficient team with decent gear could pull significantly faster than that. Especially mages, you can pull 10+ mobs as long as your tank can hold threat (hopefully they fix battle shout before beta ends.) As you start to outlevel the mobs, your XP/hour actually increases because they do a lot less damage so you can pull more as long as your team has solid AE damage.

For me, the conclusion will determine how I balance 3 leveling tactics when below level 50:
1) get and complete most dungeon quests, including completing their pre-reqs

I don't even think most dungeon quests are worth doing unless they're fairly convenient to pick up. You have to walk all the way to the quest giver, do all of the prereqs, travel to the instance and then walk all the way back to the quest giver. This can take hours in some cases. You'd earn a lot more XP/hour straight grinding. I just use wowhead to check all of the quests in a dungeon and pick up the ones with rewards I really want.

One that springs to mind is the, "In the Name of the Light" Alliance SM quest. You have to run around the entire world to pick it up but it has an awesome weapon reward so I'll probably grab it. I suppose at least you get a few extra flight paths learned.

50% more XP is BIG. Were you able to rule out rested XP? (Also, I used your figures here, expected 128 for 37 elite in group of 5 to confirm I'm using the same formula for my calc above.)

It was on Ebbnflow's stream after Blizzard raised the level cap on beta. Grinding SM from 30-38 took around 8 hours. He couldn't have had any rested left by 37 but it's possible the XP rate is increased for beta testing purposes, I suppose. Haven't seen that documented anywhere if it is.