Hi Andrew . Great review. I have the same issue with having to use SDXC cards on my RX10. I got a chance to talk to a new card manufacturer at a recent photo show, who tells me that the issue with the SDHC cards and xavcs is the file system that the card uses, not the write speed. I didn't understand the full technical reasons for this, but understand that SDXC cards use a different system to the Fat32 file system on SDHC.
Steve