well, idk, whats wrong with either my simulation with simcraft or with atlantiss, but however i do the simulation, i get better statvalues for mastery than for haste with t12 hc 391gear, (its actually mastery>crit>exp=hit>haste) although the difference is rly small (everything between 1.28 and 1.36). might be, that there are bigger spikes in dps with haste/crit (40k+ at bale for example), but in average mastery should be still better, if i didnt miss sth. (tried both reforges and didnt rly see a difference in dps at bale/rag)
for offtanking, i NEVER had issues with a full haste reforge in FL, healers were only complaining about dks/paladins getting so much dmg, eventho they sometimes had full defensive gear. --> wouldnt recommend that for DS tho, saw a lot of tanks already dieing there, eventho it wasnt to the physical part of the dmg, but still: everything helps.