Notice how I used the word enormous. I don't think Coles would make an enormous difference. Of course, it would be a positive addition and we do need another offensive weapon or two. Like I said before, Coles would probably gain 250-300 more yards than Reed would, and would probably be more effective in making sure Evans isn't double teamed quite as much.
However, Coles has a reputation for being whiny and he is past his prime. Signing Coles would take our offense up a notch, but it wouldn't be as huge as some people might think. If anything, it would move our receiving corps from below average to average.