I don't think Soul is good enough, we have plenty good legendarys on protection.
I upgraded Legs, then Sephuz, because I only do HC content at the start and love to play m+. I got Soul on 970 yesterday, shoulder last week 940..
So now I have Leg/Sephuz for Tank and Sephuz/Soul for DPS, next upgrade will be most likely archimonde trinket for DPS.