It is not similar stats, perhaps you misread (or maybe I did).
It is +X PER heirloom level. So for example Bodkins would have +2 then an additional +2 then an additional +3, hence why the Added Damage Potential rises. So you go from 15 to 17 to 19 to 22 arrows for Bodkins, as an example. This is the same principle as armor increases, for example, +2 then +2 then last is a +3.
Oh, my bad then. But.... +7bodkin arrows when you start at 15... that's horribly too many. I know you use your rusbow, so a 2 slotbow, and you don't care about dmg, but more about ammo, but my hornbow archer doesn't have any ammo (or dmg, nobody complains
) problems.
Of course, one thing that could solve this, wich was already suggested by somebody else, was the "personal" heirlooms. You want to heirloom an item, you have 3 choices for the first heirloom level, 2 for the second and 2 for the third.
Example :
Arrow +1
-Large bag of arrow
-Sharp arrows
-weightless arrows
Arrow +2
-Large bag
-Sharp
Arrow + 3
-Sharper
-Larger bad