I've been experimenting with 1h/s attacks to see how they compare to my NB class abilities and bow when attacking from the rear coming out of stealth. The numbers were surprising to me. The numbers in {} are without piercing mark on the target (Ignores 75% of targets armor and Spell Resistance).

(NB)Ambush= 1419 {1209}
(NB)Concealed Weapon= 1765 {1609}
(1h/s) heavy attack = 1633 {1485}
(1h/s)Deep Slash = 1733 {1578}
(1h/s)Bash = 1366 {1244}(deadly bash passive)pre bash nerf
(Bow)Focused Aim = 1257 {1079}
(Bow)Full Charge Bow = 1108 {900}(all passives)
(DW)Flying Blade = 916 {800}
(DW)Rending Slashes = 556+620 {507+564}
(DW) heavy attack = 819 +246(off hand) {746+224}(all passives)
(2hand) heavy attack= 2013 {1833}(no passives or abilities in the line)
(2hand)Wrecking Blow = 2783{2566}

I expected more from the NB, bow, DW and not as much from the 1h/s. All mobs tested were the same VR6 mob type, me being VR7, all were done with the same magica and stamina 1900, also all mobs had piercing mark debuff on them. No bashing enchants or jewelry were used. Bow and sword were same VR5 level and had same damage. Deep Slash was only rank 1 as well, all the others were rank 4, so when that hits rank 4 you can expect the damage to possibly be equal to or exceed Concealed Weapon.

It looks like 1h/s is getting some bonus out of stealth that DW and bow are not receiving or that it is multiplied differently.