quest boosting is not about XP per mob it is about XP per quest. I have been boosting my wife's priest through outlands this week. She gets bored with instance boosing so I try to mix it up and do a combo of questing and instancing. To make questing competitive with instancing you really need to have good "quest grouping". Multiple quests in one area. You need to go out from base, do 5+ quests and then go turn in. The Nessingwary series in Nagrand combined with a couple of the Gardar series make for VERY fast leveling. But as they wind down you start traveling more then killing.

So yes very viable, if you have good quest groupings. Usually the first stage or two at a quest hub. Elite quests are also good because your higher level should be able to easily solo them and they are worth bonus XP and usually a blue that lets the lowbie help more.

At 65 I took her into UK. Sick, just crazy. 1200+ XP per mob no rest. On my pally I can pull through the dragon room in 4-5 pulls. Took about 15-minutes. I pulled my daughter in on her Boomkin and dropped it to 3 pulls and less then 10 minutes - did it 3 times in 25 minutes. Mobs were giving 800 and change XP with the 3 of us. I believe it is 25 or 26 dudes and 6 dragons (double xp). We would have hit instance cap so about 213k an hour at 66 + Northrend greens for DE and cloth for her tailoring. When she hits 68 we will go back to questing, probably starting in Warsong Hold. There is some very good quest grouping out there and I am pretty sure we can hit 300k an hour or so which should get her to 70 quickly.