So I finally managed to get into a situation on Saturday evening that let me pull a little more comparable data on the xp to be gained from running random instances versus just questing. What actually caused this is that I was able to get into a very good instancing group, and everyone wanted to stay together and just keep requeuing for randoms each time we finished. As a result, I was able to continually run instances on my paladin for about 3 solid hours on Saturday night.
The end result, interestingly enough, was about 300,000 xp per hour. Note that in general, I did not have any quests related to the instances that we were running, so I didn't get any additional xp from quest turn ins on the instances, which would boost that number a little bit.
On the questing front, I'm able to consistently maintain an xp per hour of between 500K and 600K per hour. So, the end result is that it does significantly slow you down to do a significant amount of instancing while leveling.
Now - that being said - let me offer two very major caveats to this. This only really applies if you're planning to level a toon to 80 and then move on to your next alt to be leveled. If this is a toon that you're planning to use for end-game raiding, then there are significant payoffs to using the leveling by LFD method. First is that it's easily possible to accumulate enough triumph badges for 1-2 pieces of T9 gear that can be purchased as soon as you hit level 80. The second point is, if you never run any instances while questing, you actually won't have decent enough gear at the point you actually hit 80 to even queue for a random heroic, so you'll be stuck running regular instances for a while after hitting 80 in order to get the gear to be able to run heroics. And you may as well get xp for doing that while you go.
So, the choice is yours. If you're looking for endgame, it'll be somewhat slower overall leveling, but you'll be much more ready to jump in as soon as you hit 80. If you're just looking to hit 80 and move on to the next toon, you're better off questing.