If you disable the Siren’s head, it removes 30 from its max willpower.
Sirens usually have 30 willpower (in the last mission they have 50) so that means that they can’t keep mind-control on the next turn (it costs 2 willpower per turn per soldier I think).
I’ve never tried to mind-control a Siren (I could have but … choice with a Triton sniper in the last mission), but the Siren might have an ability to “release mind control”, the same way our priests have.
Anyway, mind-controlling it should remove the mind-control it has on our soldiers. Why wouldn’t you want that ?
OR enable us to control our soldiers through this double mind-control. But then, you have to think about a Siren that controls your priest that controls an other Siren that controls a soldier.
By the way, it would be great if recovering from mind-control was the same as “daze”, you still get 1AP to get out of harm’s way. Assaults can at least dash if they have 4WP, but the rest are just left to die in the middle of the map.