I was running around with t6 gear some more to see what can be farmed with it. And honestly it isn't that bad. Death gaze after seek range reduction (so it no longer travels to another screen instead of hitting targets in front of us), is pretty decent. You can clear fauztinville with some potions, duncraig dies relatively fast.
If you craft some random axe with max damage roll (proc isn't important), your damage goes up by quite a bit:
T6 does have 10cb and 20% slow which isn't bad, so comparing just raw damage isn't everything.
2h axe farming specs definitely kill faster than iron spiral, untill you get su setup and stack up that crushing blow. Then ability to constantly whil through crowd is really good, you gonna hate walking with axes.
But in the end, if I wanted to build a pure farming spec for barb, I'd aim for snake thrower with block, hard to beat ability to one-shot crowds with stampede. You also get max slow, max block (less defense though and less dr, need to get dr from gear).
Overall for long farming sessions you don't want build that has to left-click everything to kill, instead just drop big AoE once per screen that wipes everything, without having to aim specific monster.
2 seasons ago when I played my barb on realm, minmaxing that AH spec for heroics, snake thrower was the build I ended up using for farming, and merc barb, but that is now nerfed (elemental totem and abjurer nerfs).