I always thought you can upgrade guns no matter what with a WS.
Anyways, no dex doesnt help, neither does luk. If you're a WS, job lvl 70 will give you 10% advantage over the NPC, but thats as good as it gets.
For every job level as a WS higher than 50 you will get 0.5% more success chance at refining weapons. Only with the Weapon Refine skill, though. So yeah, at job level 70 you get 10% more success rate.
(Austrian accent) Ja, what Dee said.