I've looked at Stb_Yaw_P and have changed it from 4.5 to 3 but am not sure if I'm changing the right parameter for what I want. I want to just slow down yaw for slow panning when I film. At 3.0 it's not slow enough so I'm wondering if this parameter just gives more power to the stabilizer to hold the yaw based on the magneto.
Yaw axis stabilize (i.e. angle) controller P gain. Converts the error between the desired yaw angle and actual angle to a desired yaw rate
Alternatively if you have a programmable TX (such as Taranis) then you could put in a variable rate on the Yaw stick that is switchable. So have it normal in Arducopter and in the TX until you want to start filming, and then hit the switch which will slow it down in the TX. Same result, but then you chose when to have it or not.