Have been trying to get this on an alt this week, it's a little tricky. A lot of pugs I joined were doomed from the moment they listed in LFG, but I ran my own and on pulls 1, 2, and 3 got it to 6%, 7%, and 18%, so it's definitely in the realm of puggability for ~950 toons.
More so than any other fight this expansion, though, I'd say there are times where a player's death isn't necessarily their fault. Fulminating Pulse + Chilled Blood can go on the same player, which can be during Aman'Thul's or a storm with no RNG safe spot nearby. High ilvl does help quite a bit though as the extra health is money on this fight.
Storms during the Norgannon adds...I don't want to say it's RNG because those adds can be CCed immediately in place, but it's tough for a PUG to have that level of organization. I also think the fight would be better tuned if the order of the Torments was fixed, like on mythic.
This will probably change rapidly, but I have not found that an Argus kill on the toon they bring to be that good of an indicator of performance on that fight. A kill of Coven is much more meaningful IMO, as Coven is arguably harder (and certainly harder to PUG).
The most reasonable expectations I've come up with so far are:
--A voice chat
--After the first set of adds is complete, everyone needs to be north of 1.2M DPS.
It's tricky to judge DPS because a lot of people open super strong and fall off. Meanwhile in my run a shadow priest started at #10 and was #3 by the end of the pull. Interpreting the meters is a little challenging for a PUG RL as well because when people die at random points it's hard to compare apples to apples. I was also trying to look at damage done to aman'thul's, but realized that's susceptible to padding on imminently-healing adds.
Because pugs are so fragile I think you're better off skipping the third tank more for psychological reasons than encounter ones. It's easier to run something like 2 / 5 / 15 because there isn't that much AoE damage going out, but since 3 fulminating pulses and 3 chilled bloods go out, it's easier to have 5 healers tossing out efficient heals on them instead of 3 healers sweating it out. And with 5 heals it is not challenging to keep the tanks up. Although I technically don't know how much the absorb scales with raid size.
I've thought about trying to run something crazy like 2/2/6, but actually killing the adds fast doesn't really decrease incoming damage that much. Maybe a really great group could kill 3 Aman'Thuls adds instead of 2 in the first go...but that wouldn't make much difference.
But if anyone has any other ideas on how performance could be judged quickly or better yet, predicted via armory (at least from toons that would actually need the kill, like ~950s that are 8/11H), post!