Let me make this as simple as I can put it while properly explaining my point.
I think Archery suffers from a problem that armor has. With armor you can not only heirloom your torso but also your gloves, thus radically increasing the protection that you have VIA heirlooms by doublestacking an effect. Standard weapons on the other hand you can only heirloom and are unable to stack another weapon to further increase damage, same with horses and shields (no other heirloom will stack at the same time)... with the notable exception of archery (and the aforementioned armor).
Archery has the ability to not only heirloom your bow, but also your arrows. At a glance this seems perfectly reasonable as every item should be able to be heirloomed, but I believe that it is done in the wrong way. Both heirlooming your bow and heirlooming your arrows will produce additional damage. Where as other weapons offer +3 damage, Archery can gain +3 damage from the bow and +2 damage from the arrows for a total of +5.
Proposed change
Keep bow heirlooms the same, but change arrow heirlooms to only increasing quiver size:
Old Heirlooms (Potential Damage Bonus Per Quiver):
Arrows +3 ammo/+1 damage/+1 damage (47/54/81/108)
Barbed Arrows +2 ammo/+1 damage/+1 damage (80/88/110/132)
Tatar Arrows +2 ammo/+1 damage/+1 damage (90/100/120/140)
Bodkin Arrows +2 ammo/+1 damage/+1 damage (30p/34p/51p/68p)
New Heirlooms (Potential Damage Bonus Per Quiver):
Arrows +3 ammo/+3 Ammo/+4 Ammo (47/54/59/67)
Barbed Arrows +2 ammo/+2 Ammo/+3 Ammo (80/88/96/108)
Tatar Arrows +2 ammo/+2 Ammo/+3 Ammo (90/100/110/125)
Bodkin Arrows +2 ammo/+2 Ammo/+3 Ammo (30p/34p/38p/44p)This would help balance the power of high generation archers by keeping the damage bonus better on par with other weapons while ensuring that there is still incentive to heirloom arrows.
Effectively, this would allow higher armor to perform better against arrows as each projectile hits with less power. This also means that Bodkin Heirlooming has a significantly smaller effect, as while with the old system gaining two additional cut damage is minor, with bodkins gaining two additional pierce damage it is significantly more powerful then it should be. This new change would allow armor to remain competitive while slightly increasing the longevity of archers who can manage to stay alive.
EDIT:
For clarification is is + X Arrows per heirloom level, so using bodkins as an example, your quiver size would be 15/17/19/22 from normal level to triple loomed level.. This is why the potential damage numbers keep rising.
Another alternative worth considering
Considering that this would actually buff the archer overall (less damage per arrow but more arrows) I'm still gonna support it. Bow OR arrow looms should increase the damage but not BOTH. What ToD suggested is reasonable and gets my support.
Another way would be for Arrow looms to increase damage (total of +3 at MW and no extra ammo) and instead change the bow looms to increase fire speed/shot speed/accuracy (pick one). That would also be a way to balance it. What do you say?