Hi Cedric,
thanks for the screenshot.
Yes, it is normal.
Since you have monitors connected, you have correctly enabled the Priority for both GPUs connected to monitors, and set it to LOW, to leave a portion of GPU power for handling system/monitors.
This is good to have when you are editing the scene.
You can set The Priority popup to HIGH when you need the full rendering power, and the machine is used for rendering only.
@ cjadams: the devs are working on PR10 plugins, just a little bit of more patience.
Happy Mac GPU rendering,
ciao Beppe