Zephyr's main draw (archery pun) isn't its damage output. Damage-wise, per shot, it will always be lower than Daedric. The benefit of using Zephyr is that you can shoot faster than you can with almost any other bow, and it benefits from the Quickshot perk like every other bow. This means you get better DPS if you fire frequently.
It's a Dwarven bow, which can be improved if you have Arcane Smithing, and benefits from the Dwarven Smithing perk. How high those improvements go depends a lot on whether you've got crazy enchanted smithing gear and crazy maxed smithing potions active when you improve it.
The value really comes down to your play style, though. Because shot for shot, if you're a "fire into melee when plops hits the fan" kind of player, you'll get more out of an improved Zephyr than you might out of an improved Daedric bow, because (at base level) Zephyr does its base damage per shot per second: 12.
A Daedric bow has a base damage of 19, but does about 9.5 points of DPS. (Skill Usage > Draw Speed > Bow Comparison.) If you sneak everywhere and deal damage based on stealth and stealth bonuses, it works off the base damage of the weapon. That's where Daedric is going to make a bigger difference.
So, it comes down to your style (and the arrows you use) more than the pure damage output.
tl;dr - Zephyr has better DPS, period. Daedric is better per single shot if you sneak, or if speed is not a factor. Both can be improved beyond ridiculous.