I think it would be best if head damage was balanced for anything above 5ps horn bow damage one shots everything but the highest 5-10 helms. That way it could still 1 shot most people with a headshot, but the toughest helms would protect from it. Unless it's something like a long bow/rus bow with 8ps... that should 1 hit headshot everyone. I feel like this is probably close to what it is right now, so I think it's fine as it is.
Couched lance to the head isn't even guaranteed 1 shot (trust me, enough people have survived that attack for it to be mentioned)
Really, no single attack from any class or weapon on this mod should be "guaranteed" to 1 hit headshot an enemy, that's just a ridiculous idea rendering any kind of helmet or loomed helmet useless especially seeing as there is so much ranged about too.
Lets either do a) what momo said:
It needs only headshot dmg nerf and it will be fine. Like 10% dmg nerf, then head armor may be usfeul again.
or b) give my old friend
chers better melee options so they can stop kiting/headshotting etc etc.