How does using enchanting to increase weapon damage compare to smithing? What I mean is, if I make enchantments for increase damage with a bow by x%, how does that compare with just smithing your weapon for extra damage?

If you use Alchemy, you can make a potion that does more for the weapon than a Fortify X would.

I'm sure that the "Fortify X" would do more without Smithing though.

I am not going to drop perks into alchemy though. I am wondering if I use enchanting to increase archery damage, will it do a decent job? It doesn't have to be as powerful as smithing - which I think is OP anyway.

Probably all boils down to preference.

Increased damage by smithing is 20 additional damage points with the perk (x2 modifier) at Legendary. Not sure about the Enchanting %.

It's all described in the USEP. Look it up.