This feat is exactly the same as the 'swarm of arrows' epic feat (in the srd). It'd be nice if you could change it. Perhaps allow epic range associated bonus feats? Or perhaps a feat that doubles the amount of shots granted by many shot/rapid shot/improved many shot? All of these choices offer about the same balance.Hail of Arrows (Su): At 20th level, as a full round action, an archer can fire an arrow at each target within range. Each attack uses the archer's highest attack bonus and each enemy may only be targeted by a single arrow.
Also the feat isn't that overpowered considering arrows hardly do any damage whatsoever, compared to say, a widened fireball that could deal 36 damage to everyone within sight (if the wizard has fire resistance or is flying overhead, ie.).
Maybe a feat that allows you to combine swarm of arrows and shot on the run to allow swarm of arrows to become a move action (but prevents any further attack actions) and instead allows movement, as well as attacks on new creatures that appear within range after movement?
Just some ideas.