I would say that we should add a STR requirement to the Mighty bows... Perhaps STR 10 for +1, 12 for +2, etc. After all, they're harder to pull, which is why they do more damage.
Why not have mighty +5? Well... What primary archer is going to have a STR of 20+, even buffed?
In NWN with Mighty bows, I'm not sure how it works with bonuses above one. For example, a character with 14 STR using a Mighty +3 bow... Would (s)he get +2 damage, or nothing?
Also... I would think that even Mighty +1 would use Iron rather than Copper. Just my opinion - it's a tougher material to get, and, after all, it's a +1 material.