IIRC from "the literature" 1.120 is the threshold above which osmotic pressure becomes a real concern.
I think the term you're looking for is "vitality." Viability is a go/no-go thing, either the yeast is alive or dead. Vitality is more a measure of the health of the yeast. For example, you could have 100b "viable" cells that are sickly, and you could have 50b cells that are healthy. The 50b healthy cells will have better fermentation performance than the 100b sickly cells.
It's like sending one million grandmas into battle vs ten thousand young, healthy soldiers. Who do you think will win the day?
I've gotten S-04 up to 13% ABV, so the alcohol tolerance really depends on the vitality of the yeast, how you handle it, pitching rates, etc. But under the right conditions, you can get up into the 15-16% range like Avery does.