And a holy priest is not a bad farmer. Just get the good talents to help you. Get Spirit Tap for mana, get Surge of Light, Searing Light etc to improve your smite. Get Improved Divine Spirit for even more damage. That is a spec that can and will let you farm, and be a good healer. I've had it while rampaging Karazahn and Gruul. Beyond that, you'll have to get to a more appropriate talent build, but then you'll also have the improved gear to support the change.
I still have all of those, except spirit tap, and i do very fine in SSC/TK (except you can't have spirit and CoH)
FYI: i have over 800 +spell damage, and i do just fine. 1,3k smites, 2k holy fire crits etc.