I'll crunch the numbers for normal Guardian (highest Armor) and Mage (lowest Armor)

Normal T8 Guardian Armor gives you 261 armor ... 72.30% mitigation

261 - 15 = 246 ... 71.10% mitigation @ 1 stack of Sunder Armor (1.2% more damage taken)

246 - 15 = 231 ... 69.78% mitigation @ 2 stacks of Sunder Armor (2.51% more damage taken)

231 - 15 = 216 ... 68.35% mitigation @ 3 stacks of Sunder Armor (3.94% more damage taken)

So those 3 stacks are making that target take 3.94% more damage. I'm failing to see how this makes up for the Dagger getting almost half the damage per energy of the average melee weapon.

Normal T8 Mage Robe gives you 113 armor ... 53.05% mitigation

113 - 15 = 98 ... 49.49% mitigation @ 1 stack of Sunder Armor (3.56% more damage taken)

98 - 15 = 83 ... 45.36% mitigation @ 2 stacks of Sunder Armor (7.70% more damage taken)

83 - 15 = 68 ... 40.48% mitigation @ 3 stacks of Sunder Armor (12.58% more damage taken)

While it does provide a higher damage bonus, a 12.58% damage increase is not going to come close to overcoming energy costs that result in almost half the damage per energy of the average melee weapon.