Since I installed the 4K update on the Sony x-70 (Ver 2.0), I can still successfully import XAVC-L (HD) clips directly into FCPX (10.2.1) as I described in the post above. However, for some reason, I am unable to directly import the XAVC -L (QFHD) 4K clips. I can however, read and transcode them using Catalyst Browse. The transcode options in CB include ProRes, but only at highest resolutions of HD, not 4K. However, CB will transcode the 4K clips from XAVC-L to XAVC-Intra at 4K resolution, which seems to be the only way that I can get the 4K clips into FCPX. The resulting XAVC-Intra files, still *.mxf, at least in my tests, are 5X bigger than the original XAVC-L files. But they do ingest normally into FCPX. Thus far, these clips seem to edit nicely and I have no need to create ProRes optimized files.
When I attempt to import the 4K camera native files directly into FCPX from the card using the FCPX import screen, the audio track is visible, but the video appears as an unintelligible green screen. All of my attempts to directly connect, archive, and copy PRIVATE, XDROOT directories, etc., around as alternatives have proven fruitless. As I said, XAVC-L clips of the HD variety are ingested perfectly since the Ver. 2.0 update. These HD XAVC-L clips also co-exist comfortably with their QFHD cousins on the card in the same private/XDROOT/clip directory, and their presence or absence seems to make no difference in helping solve the 4K import conundrum.
I have also tried re-updating the ProVideo Formats as well. That seemed to make no difference.
If anyone has successfully ingested the 4K clips from the card, I hope that they can share their success story.