Settings in BloodSim are taken from Zarko's excellent thread on tank stats at Tier 11 (World of Warcraft - English (NA) Forums -> What we look like after one tier of raiding) and damage based on wielding 372 weapons.
- 175,685 health, 32.36% avoidance, 46841 armor, 21 mastery
- Boss hits every 2.4 seconds for 110k-120k unmitigated damage
- 100 eight minute fights
Difference in switching from 2H to DW:
- 7.9% less damage dealt
- 9.8% less threat
- 0.1% less damage taken
- 0.7% less damage absorbed
- 1.1% more healed
With those stats I can see no reason, at all, to ever Dual Wield if it results in equal stats. That's the major caveat. All of the simulations I run assume equal stats (except DW uses 3/3 SoB) between both specs.