Well lets take a look at a full 6 second rotation assuming a bossfight with enough rage. The prot-warrior can use 4 actions in 6 seconds. Shield-Slam, Revenge, Sunder Armor, Sunder Armor.
Lets see what we got:
90 white dps * 6 = 540 raw white damge threat
1 shield slam = 250 raw threat and assuming to average tanking gear including 10%crit rate minus mob absorbtion ~400 raw damage threat
1 revenge = 355 raw threat and ~70 damage threat
2 sunder = 520 raw threat
Sunder Armor has 260 threat, Heroic has 175 threat and xxx damage.
How much damage is needed to equal that?
Well thats a tricky one, it depends on weapon speed and dps awell as mob-absorb. If your weapon has the same speed as sunder armor (1.5seconds) your heroic strike damage must be 85damage higher than your white damage to equal the threat.
260+white damage=175+heroic damage
The bonus damage from heroic is 157. the mob should not have an absorb rate >45% or you won't reach that. the funny part is to lower the absorbtion you need to sunder it anyway ;)
As you can see its hard to get the threat sunder armor does with heroic and it even becomes harder to impossible with slow 1handed weapons. The faster the weapon, the better.