As ekwipt said, if you really need tremendous performance in FCPX you should use an AMD card instead of buying 2 1080ti. A cheap Radeon will destroy the best Nvidia anytime in FCPX.
I'm a professionnal video editor and if I chose a 980ti it's because I wanted the right balance between gaming performance on windows and good performance in video applications on OSX (probably your case too?)
This is correct.
FCPX is designed for real Macs and tailored for AMD which has higher computational numbers than nVidia (which has better raw performance).
I really doubt Apple tests FCPX with dozens of GPUs. It's specficially for iMacs, MacBook Pros and Mac Pros.
So if you want steady performance even over time with different OS updates, get an older AMD card like the 280x.
I built a friend a rig for FCPX and specifically got a 280x card and he hasn't had 1 issue, even after upgrades from El Cap to Sierra. This is because Apple uses these exact same AMD GPUs.
If EVER Apple goes back to nVidia, especially their higher end consumer GPUs like the 1080Ti in the next Mac Pro (next year?) then FCPX will be tailored for that.
Now Premiere is another story. Adobe has to make it work with all sorts of GPUs.
FCPX and Premiere performance are apples to orange comparisons.
Maybe you can get a 280x and keep your 780Ti in the system? I'm not sure if FCPX will have issues.
Me, personally, I have a bigger problem with FCPX (which I don't use because I don't like it anyway)....but my playback just stutters because it doesn't like dual 980Tis.