Proficiencies are a binary thing. Either you have it (Thus add your proficiency bonus to it) or you don't.
It doesn't stack.
Only Expertise will provide an additonal bonus to a Proficiency, but you cannot get Expertise for Saving Throws, only skills.
Thus, in relation to your question, the feat would do nothing as you already had DEX Saving Throw proficiency.
Thanks. I supposed it would work that way, but didn't find an article that really confirmed it. The increasing bonus confused me.
