Depends on their level. The one from Dread Wastes and Townlong Steppes have a lot more health than the one from Krasarang Wilds and Jade Forest. Either way, at level 90 you should be able to outgear them very quickly given how easy it is to catch up now.
Frost mages soloed the Warbringer rares with 2 fingers in their nose.
I soloed countless of the Warbringers on my priest. Even without root (it lasts for 1-2 sec anyway). The easiest ones were the Dread Wastes ones because of Klaxxi buffs, but all were doable. My biggest problems were terrain reset bugs, world PvP, failbots taunting and kiting it (e.g. into Sha of Anger; all were reported and GM told me their behavior is not acceptable), failing nitro boosts (but I should not rely on them anyway), and the required focus which I rather spend on raiding. After a little bit of practice it was doable, and I succeeded multiple times. If it is still possible I'll have to see. Definitely up for the challenge.
Since I'm an incredibly friendly person I'll give you all a small piece of advice on how to deal with the fear besides your obvious class abilities:
http://www.wowhead.com/item=92786 (A) /
http://www.wowhead.com/item=92787 (H). Yes, it is a DPS loss, deal with it and use a +1000 stat flask. And as priest perhaps Glyph of Fear Ward. Personally, I use Glyph of Weakened Soul to be able to use B&S earlier on.
---------- Post added 2013-03-19 at 07:26 AM ----------
Doesn't work with mobs which drop mounts. It'll become zerg. You can already see it on Sha and Galleon. Its not about performance its about tagging the boss before it is dead. Same with new rares. Also, how would you then distribute the bags and rep bonus?