UPDATE: I have done testing on the variables of armor which could possibly influence its protectiveness. What I have found is that armor level, layer size, layer permit, coverage, and material size have either absolutely nothing to do with protectiveness, or have a negligible impact on it.
I found this by increasing these values to absurd amounts (excepting armor level, which I just put to 3) on the mail shirt in version 47 in the armor items raw, and then attempting to pierce through the armor with a steel spear using a pair of spawned humans in the object testing arena, both with the same equipment (iron mail and a steel spear) as well as the same skills (max fighter and spear user). The result, every time, was that the spear went right through the mail, tore the upper or lower body of the targeted human, and damaged the mail shirt itself.
By absurd amounts, I mean degrees of magnitude. Two of them, in the case of layer size. There is no apparent difference between a mail shirt with layer size 15 and one with layer size 1500.
The obvious conclusion is that the only ways to change an armor's effectiveness are either by making it chainmail, which makes it worse of course against blunt weapons, or by of course making it of a stronger material. There is apparently no way of making one kind of armor weaker than another by any other methods. Though, using the layering system, and making some armors shaped, you can still influence the strength of worldgen soldiers, for instance by giving their civilization only shaped armor, and no chainmail equivalent.
This is indeed exceedingly disappointing, as I was hoping for some method of creating differing effectiveness for different armors, but oh well. Though, you can still utilize layer size to make some armor heavier than others.
Please do tell me, anyone, if my !!SCIENCE!! is flawed to any degree, or if you have anything to add to the conclusions reached.