Overall though, I doubt most people would be respeccing to run lower gear instances, and if they did, I think you'd find you could get similar results from going full arms or fury if you're confident in your gear and skill, for these types of situations.
As far as your spec goes, i would try to not go so far into prot if the intention of this is to try out another tanking style, and perhaps go as far as to get bladestorm. Maybe Talent Calculator - World of Warcraft ? I'm not sure, though. I'm of the opinion that you should go at least 51 in prot, or not at all.