To be honest with you, as an experienced archer, using a longbow and 160 wpf it is more than viable to aim for the head reasonably accurate at a close distance (the one that was in the vids). There is a degree of randomness in it at longer ranges of course but as long as you smoothly lead you'll have a decent chance of getting off at least one arrow towards the targets head.
I was still able to pull off accurate shots yesterday with the set up while wearing Black Armour & Greaves (Straw hat & Mail Gauntlets) so accuracy isn't a problem too much with light armour with any bow however I created an alt, who had 5 power draw and as much archery as possible and I have to say, it was very accurate and still had a decent chance of 1 shotting the person to the head. I think when most people talk about a "quick pew pew archer" they are talking about people with horn bows, they're a lot faster than the warbow and particularly the longbow with more accuracy.
Pulling across someone with Masterwork Bodkin arrows & a Masterwork Horn Bow with 5 Power Draw and 160 wpf, they're going to be quite accurate across medium distances, more than likely shooting at targets that don't know they're there so that they can get clean hits.
The pierce damage against the heavier helmets will help significantly in this case, someone with a Barbutte for example holding onto 50 head armour will be quite squishy with the current loadout.
Bodged and broken calculations I was going to use as evidence however I need to actually know the statistics behind what I am saying. The numbers are all guesses from previous knowledge and have probably moved on quite a lot. For example, I thought PD increased damage by 8% per level, the wiki says it increase it by 14%. But all the same, i've done the calculations if you want to look, it's roughly correct for the most part, at some point in time, whether or not i've created it from thin air!
Also, afaik, these are best case scenarios. To be brutally honest with you though, I think what I have written in this spoiler is a load of bollocks.
Now, my figures are going to be off somewhat here, but i'm sure this will get you the general jist of what I am trying to say;
Masterwork Horn Bow: 26 Damage
Masterwork Bodkins: +2 Damage & Pierce
5 Power Draw: +40% Damage (+70%)
Total: 26+2=28, 28+40%=39.2 pierce damage. (47.6)
Headshot: +125% damage therefore;
End Damage before Native multipliers == 88.2 pierce damage. (107.1)
That's nice and high and is sure to make a lasting impression onto the soldiers head even if they are running around with decent armour and i'm sure my calculations are wrong somewhere along the lines (particularly because I believe there is another basic headshot damage multiplier that I haven't added this calculation & I haven't done any armour soak etc. Also I haven't added wpf damage increase either. I THINK that would give but i'm pretty sure it's wrong.:
160 * 0.14 = 22.4%; 88.2 + 22.4% = 107.9568 pierce damage (131.0904 pierce damage)