Do you have insulation in the roof, how much, that is a first concern
affecting your usage winter and summer. When I had AC retrofitted I had
a second floor return added, I am able to reduce 1st floor suplies and
returns by 60% or more to force AC upstairs. Ive checked the temp just
over my cooling coil and have little difference as a result of reduced
airflow. Your set up was for heat before adding AC, heat rises, you can
rebalance your system a bit to help also leaving your fan on 24x7 wil
help alot. Just static venting a cathedral may not be the issue or help.