Oh, another thought. If you insist on splitting the attack/damage bonuses from the bow and arrow, I'd do it the other way around: attack bonus to the arrow, damage bonus to the bow.
Reasoning (acknowledged: shaky ground due to applying anything real-world to D&D) -- quality of arrows affects the accuracy of archery far more than the quality of a bow. With the same bow, going to a higher quality (higher +) arrow won't change penetration (damage) much, but will improve accuracy. Likewise, going to a better quality bow with the same arrow is likely to increase the energy the arrow retains, which more readily translates to more penetration (damage) than more accuracy.
'Course there's the whole accuracy vs. penetration thing to consider for damage, too, so -
YMMV.