optimised media - x2pro - avchd...how can I tell FCPX to point to the optimised ones not to the original media?

FFCP always uses optimized if it's available. The problem is you want X2Pro to use the ProRes files. From the advise you got from the X2Pro forum it sounds as if you cannot force it to use the optimized files rather than the original files. Normally when you import AVCHD media it's rewrapped to QuickTime during import so the problem doesn't arise.

SSelect all the media in the browser and transcode. FCP will only transcode wgat isn't there already.

THere are threads in threads in the first message. It sounds like they want you to remove the mts files and somehow use the transcoded media.

PRoxy and optimized have the same audio quality.

I think X2Pro assumes professional production media to start, which .mts is not.

I'm glad you got things going. I should leave it at that but I can't because your message is so misinformed,

AVCHD is a professional standard given that is used by XDCAM and P2 cameras...

This is wrong in every respect. First cameras that use P2 media use primarily multiple Panasonic formats, DV, DVCPRO, DVOR50 and DVCPRO HD, that are both SD and HD. These formats have absolutely nothing to do with AVCHD, which is H.264 MPEG-4. The Panasonic formats are I-frame and use a completely different encoding structure, relying on raster resizing in HD for compression. AVCHD is MPEG-4 based and XDCAM is entirely different, an MPEG-2 based format that uses an MXF wrapper. XDCAM only uses MPEG-4 for proxy files. The MPEG transport stream was intended to be used solely for broadcast transmission and was later adapted for acquisition. It was never intended to be used as a production format. Many users began ripping the transport stream files out of the AVCHD wrappers, which led software manufactures to allow including these transport files in their applications. This was a grave mistake in my view and should never have been allowed. There were incredibly simple procedures to directly import AVCHD media into applications that rewrapped the MPEG-4 files into a production format. That should have remained the standard, but unfortunately it was not, and some users ended up with this completely inappropriate transport files inside production applications.

If there is anything that MUST happen it is that Avid MUST abandon the totally obsolete and very limited OMF function, and it's almost equally arcane AAF function and join the 21st century and start using far more comprehensive and versatile XML functions.

known problem of FCPX with relinking compound clips, while switching from .mts to .mov

This only happens if someone very unwisely brings .mts files into the application. This should never happen, and would not happen if the media was properly handled.

And P.S. I think when X2Pro support said use High Quality media, transcoded, optimized files is exactly what they meant. When a library uses managed media it puts the optimized files in a High Quality folder and the process in a Proxy folder.