My take on the helmets and armor in the game.

The only unknown is what helmet type they have, and thus the HS reduction stat.

It is currently still optimal to aim for headshots, no matter what helmet type they have, as even with tier 3 helm, a HS is still just as much damage as a body shot.  Any lower tier helm and you're doing bonus damage.  Knowing how many hits is required to knock someone is going to be dependent on how comfortable a player is with their skill level, and being able to mentally process the damage feedback quick enough to either adjust their aim accordingly, or realize it is going to take longer/shorter to achieve the knock