I know people import and edits lots of FS5 and FS7 material (inc'l higher frame rate) using FCPX. I don't know about the Z90.
Looking at the Z90 manual, it appears 100 fps is a special high frame rate mode such that when that is ON, then selecting 1080/50i actually records 100 fps.
I don't know why it only imports as audio, maybe it needs pre-processing with a Sony utility? Another possibility is might need all the metadata from the folder tree. Are you importing from there or just copying the video file itself to disk and importing that?
If you can upload a short clip of this -- including all surrounding folder tree and metadata files -- to a downloadable site, I'll be happy to look at it.