Good point. Will drop Faster casting for chaos damage.
Try drop it the other way around. Drop Echo for Chaos damage. Especially once you have vaal haste. You will max stack quite easily with faster casting alone, and that way you dont sacrifice the 10% Less Dmg. As well as it gives you a good amount of flexibility in your movement, because you never get animation locked for too long.
As Archer said. having more damage per stack means that you need less stacks to do the same thing. Trash will require 6 stacks instead of 10 for instance. And still managing to reach max stacks when it matters will mean that your top end dmg is much higher than it would otherwise be.