As the stats work currently, heirlooming throwing daggers gives precisely the same benefit as heirlooming heavy throwing axes. All throwing, in fact, gains the same modifiers. These modifiers are pretty good for the low tier items, because they accompany larger ammunition amounts. 25 cut becoming 29 for 8 Daggers is a rather good boost. However, 44 cut becoming 48 cut for three axes is not nearly as nice. Throwing weapons are unique in possessing a very limited damage pool, while archers and crossbows are also limited, their limit is generally much larger.
Previously, heirlooming throwing gave "large bag of balanced" instead of "masterwork" as the final modifier. I would suggest a return to this for the Francisca, Throwing Hammer, Throwing Axe, Heavy Throwing Axe, Javelins, Throwing Spears and Jarids. For most of these, it would provide 4 ammo, 5 for throwing spears. It would reduce the total damage per hit of +3 throwing weapons in this tier while at the same time increasing the player's total damage potential for a round of play.