If the light armour heirloom bonus were increased to +7, then between gloves and padded leather a 29 body armour suit, which is what I wear, could become 43 armour - which is an increase of 48%. It seems overpowered to me that two low-tier items could have such an enormous base stat increase. On the flip side, there are suits of 72 armour getting a 14 point stat boost, taking them all the way up to 86. At this level players become tanks who are really no fun to fight (for me).
Considering this, I voted for changing all armour heirlooms to a total of +4.