I have been testing a little too, both C 9 pro and Wavelab pro 9 (on a macpro mid 2012), and C 9 elements (on a imac late 2009), with waves, plugin-alliance, ikmultimedia, native instruments, and other plugs, and everything seem to be working as before. I have the system on a SSD, but the homefolder (with the plugs) and the production disk are on a regular drive, so I do not benefit much of the APFS in day to day use. Although everything on my system is not supported on HS yet, it seems ok here.