Truly, I understand your points very well Gauntzero! and please, don't take my musings to be anything more than theoritical ramblings on a discussion.
To me, I find it isn't that powerful as it's use is primaryly stationary, and despite Komissark missing the point that a higher agility character can just run out of the 45 degree firing arc before the overwatch shot is launched (resulting in a miss), it can be mitigated fairly easy. The things that tame Overwatch are the following:
1. Have to be stationary.
2. Higher agility characters can just run past your killzone before you fire, resulting in a miss!
3. There's a **** ton more of the cultist with autoguns than they are you and your fellow acolytes.
4. As a gmnpc'd cultist, i can just throw grenades blindly in your general direction or use smoke grenades to prevent your triggering of visual based triggers.
5. There are tons of gear options that limit viability of other trigger actions based on sound, etc.
6. The npcs can make a pc waste his turn by standing still or doing nothing at all. Wooooo