If you ask on the general discussion forum for the healer spec dps rankings, I’d bet someone could analyze the Warcraft logs and give you the exact rankings. Nice breakdown Zipster.
If you exclude disc priest, how well you heal on the class is the most important factor in your dps. If you are a great and efficient at healing on a class, you would have more time to dps. The better you heal the more time to dps. In raids. It’s a competition to be the highest HPS healer so you won’t dps much.
PS. If you own BFA, you can make 110 trial characters and test all the healers in the tutorial, dummy, and 15 min battle scenario. Use the Icy Veins class guides for Rotations, priorities and talents. Leveling a few classes to 30ish would help if you can’t make trial characters.