Well, since you already have a coop, I guess how much work are you willing to put in to modifying it? :p How cold does it get where you are? If you could cut some ventilation holes, that would be better, but I don't know if that's feasible.
Also I love the sign on your coop. :)
More ventilation would be better - remember that if there isn't enough ventilation, moisture will build up and lead to frostbite in the cold. Where are the roosts relative to those windows?