My current Spec is designed around increasing my damage as much as possible while allowing me to tank effectively. I am my guilds back up MT and main Offtank. One of the reasons I use this build is that it lets me PvP and farm in my tanking gear fairly well. I have a Block Value set, but sadly the armory isn't showing it atm, ~600-700 BV, 23% crit, 12k hp.
My guild has downed Kara and Gruuls lair. We even killed Mag once but have yet to repeat the feat.