2 shot? I've seen chainswords do that too. And from what I've seen the witchblade and sorcerer's sword are around that same area in damage. I assume you hit wounded targets since usually it takes around 3 hits for me to down a target with my witchblade. Or you know, you are a scumbag marine, making crap up per usual.
I can only speak for my ork slugga but for one thing, their power attack aren't faster than our fast attack. Melee is easier vs chaos than vs eldar as an ork.
Mods can make powerswords equivalent one shot from what I've heard, though I can't confirm myself since I have no mods for melee weapons and hence can't test it myself.
Thus far I have not had any moddings for my witchblade. Though, thats pretty rough to deal with if they one shot and goes against their design. If they want to give warlocks a one shot heavy weapon, I'd wait more for singing spears to be their heavy wep. Or maybe something like a executioner witchaxe, thingy.
its also ALOT easier to get behind people as eldar which increases your damage, plus you know, power sword vs my apocs chain sword doesnt help, and the eldar melee feels ALOT faster, especially the parry. i havent got any power weapon mods, but i have got multiple chainsword mods to increase its damage - not that it seems to make any difference - or shows any more damage, but oddly when mousing over a bolter it shows the dmg increased by 10, still doesnt seem to make it anywhere near as nice as a power sword.
Use the adrenaline heal (you can have twice the buff) you hit from the bacl for 449 and the front for 299 with the chainword dmg mod. You one shot almost anything.