I've found mighty bows a bit strange because no matter what your strength is your arms are only so long therefore you can only pull it so far thus there should be some limit on a normal sized longbow.
Actually on a mighty bow, it isn't the distance your pull it, but the actually strength required to pull it. Assume all bows pull the same maximum distance, but the string requires certain strength to pull it to that maximum distance.
So a bow with a STR+4 pull can reach maximum pull with STR+4 or greater, but a STR+2 can only pull it so far and thus damage is reduced.
Personally I would also reduce the distance the arrow can travel but that's another story.
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In regards to crossbows and "mighty-ness", the reason strength does not play an issue is that a crossbow is winched back. The heavier the pull (mighter in other words), the bigger the pulley for the winch, and the longer the time to load it. Any person can winch the thing up, their strength has nothing to do with winching unless the pulley ratio was quite small.
Strength may be a factor in speed of loading, and thus a "mighty" crossbow would have load times dependent on strength, however its damage would be fixed.
One alternative system would grant crossbows equivalent DR due to the nature of its design. The bigger and mightier the crossbow, the more DR it can overcome. Or perhaps it can ignore certain amounts of AC from armour. Perhaps 1 point per +2 strength.
D