I think you should not compare the damage increase purely based on the increase in iLvl, but compare it to other weapons from the same iLvl. For instance comparing the Zeta with the Dreadwyrm. The base damage of both weapons are the same, so the secondary stats of the weapon should determine whether or not the weapon is BiS for you.

The Item Level is just a number that indicates which “tier/grade” of content the weapon belongs to.