The purpose of the RoRG, despite worse rolls, is to enable the set bonuses. With it, Aughild's, and Blackthorne's, you will reach +25/-25 damage to and from Elites, as well as several other bonuses.
That is far better than the Unity, which does not even have a CHD bonus on it, assuming you aren't just using Unity to cheese solo with a follower.
As for Elemental Exposure, it will perform just as well if not better than Unwavering Will. UW will give +10% damage only after standing still for 1.5 seconds. EE can easily reach +15% damage immediately and constantly. That bonus is applied to everyone in the group, as well, making it very valuable. Arcane Dynamo is not bad either, if you're constantly throwing out meteors or explosive blasts. Dominance is not bad for trash or dense maps, but weak against single targets such as Rift Guardians.
Yes, make a Devastator with 20% fire bonus (or as close as you can get, obviously) and something like 1180 minimum and 1450 maximum damage on the elemental damage range. When you do, notice that the white damage range numbers are actually higher than Sever. These numbers are what your skills use as a base. That means they will do more damage with the Sever, without even counting the element damage bonus. It will be significant and noticeable.