World of Warcraft Various Guides - Tanking with a Vengeance Guides
That would mean that at the full 10% stack it would be 4,500 extra attack power. Knowing that at level 80 it takes 14 AP to grant a 1 DPS increase it equates to roughly a base 300 DPS increase. Past this number it is hard to figure out exactly how much extra real DPS and therefore threat you would gain since it depends on the abilities you use, their coefficient to attack power and the threat they generate.
That is a long time, especially if the fight uses any kind of tank swap mechanic. Think of Saurfang with a tank swap roughly every 20 seconds. You would almost never get to the total bonus, and once you did it would likely fall off before it was your turn to tank again. In fact depending on how long the stack lasts, I’m assuming something like 15 seconds, it is entirely possible (although unlikely) that it could drop off while fighting an extremely slow hitting boss. At 15 seconds it simply means a combination of 6 misses, parries, and dodges in a row against a 2.5 second per swing boss. It’s unlikely at only roughly a 0.5% chance with a combined 40% mitigation or a 1.5% chance at a combined 50% avoidance, but it is still possible.
Even better, because it is based on a percentage of your health it is extremely easy to adjust as required without redoing any gear. So if tank threat ramps up in the expansion and no DPS is anywhere close to us, Blizzard simply lowers the percentage to 5%. If DPS threat ramps up and tanks just can not hold threat at all, Blizzard simply needs to raise the percentage to 15% or 20%.