Enchanted throw still plussable, but no positive effect is known.
The effect when upgrading enchanting throw is that it upgrades the dmg of vengeance sting (the hit that the attacker takes when he attacks u). Although description of vengeance sting shows its passive and non-upgradable and fixed dps (around 50% i think), the dmg u do at retaliating attacks increases according to lvl of enchanting throw skill.
I yesterday rolled my 3rd class from war to mage, and did full tests before upgarding each skill, so this info is validated in combat.
And yes, the throw dmg part of enchanting throw is exactly the throw dmg at lvl 0 of throw skill, no matter what lvl u have upgraded throw.
Fin r/s/m