QUOTE(DJNoni @ Nov 1 2018, 16:43)

You know about
abilities, right? Even if you use ability points for cloth armor, and you slot the abilities for cloth armor, you don't get the abilities bonus unless you use only cloth armor.
Ah! That's what I was missing. At this point (I'm only level 143), the trade-offs were close enough that I didn't catch them.
With my cloth abilities and my proficiency, switching to an all-cloth armour scheme gives me about 60 more MP's, and slightly higher spell hit (118.5% --> 125.2%) and crit (15.1% --> 18.7%) chances. My base spell damage is also higher (917 -->946), which is important for a mage!
Sticking with the Shade Helmet makes my staff do more damage (1085 --> 1145), my physical mitigation is slightly better (33.5% --> 34.6%), and my resistance goes up (27.5% --> 30.2%); but these are all specific properties of the helmet not tied to my abilities, so they're to be expected.
As I gain in cloth armour proficiency, it should close the gaps in some pretty important areas.