Why You Should Prioritize Upgrading Weapons
In general, upgrading your weapon is probably the single best upgrade you can do to improve your DPS, especially as a DPS.
This is mainly because upgrading your weapons provides a substantial boost to your Weapon and Spell Damage. It is a larger boost than what you'd get from a single armor set bonus, and it is further boosted by buffs like Sorcery and Brutality.
Combine this with the Nirnhoned trait for stamina DPS, and you get a large boost to your Weapon Damage by going from Epic (purple) to Legendary (gold)!
However, for your back bar weapons, regardless of whether its a Master's Bow, Maelstrom Bow, or Maelstrom Staff, you won't be seeing as much of a return.
You aren't on the backbar long enough to see the returns on the higher Damage, and the unique effects, while strong, do not substantially increase when you upgrade those sets.
If you have to upgrade anything in your equipment, do your weapons. It is well worth the return, and you'll see a very noticeable boost to your damage when you do so.