I've seen many topics lately questioning about the subject so I did a few tests. I bought an iron bow +7 an enion bow+7 and a chitin bow +7. I tested them on iron scarecrows, 100 hits of Dark Pursuer per bow. Tested at lvl 80 with the following items: dual RE+1, PP+2, dual ROL+0, Elf Belt+1, chitin set+8.
Enion bow on average did 1628 damage.
Iron bow on average did 1453 damage.
Chitin bow on average did 1314 damage.
I will test them some more, my goal is to do it a thousand time with each to get something really accurate. Will add new results when i test some more, and i will probably test helenid xbow and Eagle's eye (accepting funds to get them :P ). I hope this will help.