As an archer running around with 40 armour I personally think the increase from 27 body armour to 31 isn't that bad in honesty.
From my triple heirloomed longbow, it has increased from 26 to 29 damage. Albiet pierce damage but the increase is the same, and this is for a bow.
Although it appears that armour in the lighter areas increases more or less in line with increased weapon damage from heirlooms, the heavier items increases roughly by double. In theory and, I know that the devs APPARENTLY hate realism as an argument (although the bow missile speed nerf was run off a realism argument
), ligher armour can be harder to reinforce without a large weight increase and since these suffer no such penalty, the balancing between them has been done well in my opinion.
As another note, having mail Gauntlets that technically only add 1 weight (0.5*2) with an increase of 7 armour, giving my total increase a value of 11, that outweights anything that the weaponry heirlooms gives you regardless of blunt, pierce or cut damage.
I would say it's a mixture but the triple heirloomed lighter armours in question, although I wouldn't mind an increase, are fairly balanced in terms of gameplay value.
(Also an increase in weight would make me slower and would make me sad.)