@Zimpzon, correct me if I'm wrong, but we should keep upgrading CZ because it feeds the multipliers for Witch Doctor, Wizard, and Zap Damage upgrade -- which are needed to help kill the higher HP enemies while they are still in the mob.
I'm on Arena 450 and still need more CZ to keep the mults working for higher arena mobs.
The only one not worth upgrading is Dagger Cooldown after lv 50.