I don't think NATs 2 piece is ever going to be worth it tbh
it's extremely difficult to maintain the higher BP, your pretty much gimped for EHP unless you sacrifice damage rolls in other slots
I haven't done the math, but even if we're critting an extra 7%, it should nowhere near makeup the raw damage/EHP loss overall
That barb you just linked made my eyes bleed T_T
That barb is zapro's, aka imthereal3377 (apparently)
It's not at all my cup of tea but for the sole purpose of extracting efficient FoM runs, it's probably about as good as you can get.
I think his reasoning for nat's set is sound. My concern with nat's set is not ehp, dps or bp's. At that gear level my only concern would be whether putting a leoric's signet and/or hellfire in place of the reflection would be optimal. But zylog and Chaz have gotten around that by using a nat's sight in certain scenarios.
zapro's theory behind his gearing revolves around reliability over peak damage. That's also why he fundamentally dislikes brawler so much. He's already stated he doesn't feel like its worthwhile for him to increase his rend damage.
Nat's set is a good fit because it ensures nearly every rend is a crit. At that level of damage, it's likely more important to have as many rends crit as possible, over marginally increasing the damage per crit. That's why you can't look at the raw numbers alone, because at a point, there's no need to increase the rend damage if its already sufficient to kill everything in one pass.