The sales results are probably additive, people may check out the book or do a Byrne-steal because of Arrow, but they are only going to buy it if the book itself is good. And there is a difference now in Green Arrow imo, Percy may have been there before Rebirth, but his editor wasn't and that person has a reputation for getting better work out of the creative teams. I think the quality angle may explain why Supergirl isn't doing very well.