Next I'll talk about the "Display P3" color space in Apple --why? will that's one of the new Wide Gamut color spaces (that is if you have a new MBP, iMac or late MP 2013 running Sierra)
it's a full range (0-255) 10bit RGB space as noted
here , since there is limited wide gamut rec2020 monitors at a affordable price there lots of wide gamut photo/p3 monitors that can be recalibrated
Recently, Apple has released new Mac and iOS devices with displays that support a
wider color gamut called Display P3.
Display P3 uses the same color gamut as DCI P3

So with FCPX to work in wide gamut you need to be in display P3 , even though you my have rec2020 color space files fcpx uses mac OS colorsync
Final Cut Pro uses macOS ColorSync technology to match the known color space of
an image or video to the color space of an Apple display, according to the display’s
ICC profile. This display profile can be set in the Displays pane of System Preferences.
To view the expanded color space in a wide-gamut project, you must use a P3 display.
When using an Apple P3 display, make sure the display profile in System Preferences >
Displays > Color is set to the default profile.
When you play wide-gamut source clips or photos from the timeline in Final Cut Pro,
the media is composited within the linearized Rec. 2020 working space (and will be
output, upon export, in the color space of the project setting). When sent to the monitor
for display, the clips are converted from project color space to the display profile with
ColorSync. On a P3 display, ColorSync will clip any colors outside of the P3 color space
to a value within the P3 color space. In real-world imagery, this clipping is rare, because
colors in nature aren’t likely to exceed the values allowed in the P3 color space.

That explains why FCPX uses P3 for wide gamut , unless you have super hi detail close to clipping and in log space P3 should be ok
but to Grade/Color Correct you will need a acute wide gamut corrected display.
In my next post I'll discuss the option to view the wide gamut video from FCPX timeline correctly for color correction & export in rec2020 & rec709