My understanding is that a lot of the relatively recent talent changes etc were to balance out the classes at higher levels. It wouldn't surprise me if some classes/specs were putting out more DPS at lower levels. At L80 it would likely depend on the situation, some classes/specs are just better with fast kills while others really shine on boss fights (which is nothing new as you likely know). I am not sure if any class/spec is truly OP at this point, but I really don't have enough experience to be sure of this. Things are constantly changing anyway.

With respect to multiboxing, I believe that some classes will always perform better than others due to the inherent key spamming involved. It is just not possible to take full advantage of the random "procs" that can make the difference (e.g., hotstreak, eclipse). At least, not that I have figured out....although, I keep trying. For example, Paladins are nice due to so many instant cast spells and elemental shamans don't seem to need as much micromanagement to maximize dps as say mages or boomkins. SPriests probably also fall into this category, although I haven't really bothered to maximize my SPriest DPS since I will be switching to holy at some point so I could be wrong. I am not sure about other classes since I never played them.