I use straight HDMI out as a third monitor on my rMBP. From the FCPX 10.1. manual:
View playback on an external monitor using HDMI
If you’re using Final Cut Pro 10.1 or later on a 4K-capable Mac with an HDMI port, you can play 4K or HD 1080 video on an external video monitor connected to the HDMI port.
For a list of compatible Mac computers, go to https://support.apple.com/kb/HT6057.
1 Use an HDMI cable to connect the external (HDMI) video monitor to the HDMI port on your Mac,
and make sure the monitor is connected to power and turned on.
2 To configure the HDMI monitor, choose Apple menu > System Preferences, and click Displays.
3 In Displays preferences, set the HDMI monitor to be a secondary display.
4 Select Scaled, and in the list that appears, select 4096 x 2160, 3840 x 2160, 1080i, or 1080p.
Note: You may need to press the Option key while you click Scaled to see additional resolutions for the HDMI monitor.
5 To select the monitor in Final Cut Pro, choose Final Cut Pro > Preferences (or press Command- Comma), click Playback, and choose the HDMI monitor from the A/V Output pop-up menu.
6 In Final Cut Pro, choose Window > A/V Output.
The Viewer contents appear on the HDMI monitor. Viewer features (such as onscreen controls
and the title-safe and action-safe overlays) can be viewed only in Final Cut Pro. Note: Video and audio are synced at the video frame (not audio sample) level.