After playing a little bit with the new set, I tried to understand/verify the calculations going on there. I do have some vanilla jewelry with +min-damage, so the numbers are quite easy to verify. Or so I thought. 
I started with the question, how the 70% more damage for HA is working, and how often an arrow can pierce. That was the easy part. 
But the 2 piece set bonus of the new set is acting strange. Maybe someone can verify that or explain the reasons for that.
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With no Momentum stacks at all, I get slightly bigger numbers than expected. At first, I thought about some missing multiplier or some additive/multiplicative error. But it seems to be, that even with no Momentum at all, you get a damage bonus between 1% and 10%. I have seen only 6 different damage numbers in that case so far. It seems, that the modifier is rounded to 10 frames (there are internally 60 frames per second, right?)
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At full stacks, it seems to be different again. Of course, I do get slightly lower numbers than expected. I (almost?) never get 100% damage bonus with 10 “stacks”, but more something like 95% and 99%, even in melee combat and permanent attacks. That would be consistent with the set description, as we get 10% per second momentum, and we never actually have 10 seconds left.
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However, I do see a lot of different numbers popping up at 10 stacks. So, there is no rounding, but maybe the damage modifier is only refreshed every 10 frames … whatever.
tl;dr:
GoD-2 always gives 1-10% more damage, even on the very first hit. But we (almost) never get full 100% more damage.