Well, it's not like you don't get extra damage from elemental reactions as well, on top of being able to use the better character. So yes, 10-15% extra damage is good, but you will probably gain more extra damage from elemental reactions and the characters you can use instead.
Swirl is by far the strongest elemental reaction in AoE scenarios.
ST however it's pretty bad. That's why Xiao "speedruns" are pretty bad (honestly not much better than Keqing teams) eve though he has insane modifiers.