I think the efficiency would be neutral AP wise, if you would use it with 2 AP attacks (2-hand or Dual wield). It gives 50% bonus, so you would deal 150% damage at the cost of 3 AP, 150% AP costs. Can be worth if it the damage would be enough do kill an enemie and you would not have 4 AP for two attacks. But is it worth one trait point?
Regardless I still finde sneaking during fight in most cases pretty ridiculous.