The problem with this, is that some lucky ppl will always abuse of these things. Imagine a seeker with a full slaying lvl weapon. Mobs will die so fast in GV, or even a sin with a slaying lvl weapon for world boss or TT. I don't think it would break the game, but it definitely will unbalance it. Like, some ppl wont stand a chance in pve.
P.S ( Slaying Damage Amplification = 3 * Slaying Level / ( 300 + Slaying Level )
damage dealt = raw damage dealt * ( 1 + Slaying Damage Amplification ) )