Bows increase in power on the down arc of there trajectory.. so if u want to go down that route..
each bo has optimum range and can shoot x beyond it but in either direction looses power ^_^.. would also make for some cool skills like high arc shot 4x power increase but -60% chance to hit and ignores some armours like plate and cloth, but power reduction to 50% gainst scale mesh and chain.
Anyway, yeah the bows in shaiya all act like compound crossbows, flat trajectory, with a optimum power point, but u cant target out of this optimum point o.O, if they did increase the range it would be with a power reduction imo.