karmic abyss
I'm not a fan of it just doing extra damage - it was unnecessary in NMRiH1 when most weapons would already kill in a single headshot, and it's another one of those things where you somehow make your bullets shoot harder.
If it's to be re-implemented in any way, it'd be better as something like holding breath in any tactical shooter - reducing weapon sway/shake so you can better place your shots - particularly if the brain-stem "weakspot" is modelled as in NMRiH1's Realism mode.