From mid 2014 may not be a big enough jump. I'm thinking on my Late 2013 as well.
Your needs and workflows are key though. Four Thunderbolt 3 ports is impressive. As someone who live streams that may be more important than any impact on FCPX.
Do you think you'll see advantages moving to TB3?
The Touch Bar will probably have greater value in the next couple of FCPX updates assuming you like its potential.