So I've installed the new firewire card (SYBA PEX30009) and I can very happily confirm that it works OOB as is supported natively with my 300 series mobo (Gigabyte GA Z370M D3H). My previous power management woes (inability to sleep, and shutdown causing reboot) that I had with the Startech card are completely gone
Audio output is perfect. I've installed it in the PCIe 1x and have not noticed any lag or glitching with my external audio interface, the Focusrite Saffire Pro 40.
......
So happy to finally have an OSX system that supports such lovely PCIe cards, NVMe, simple multi-booting, water cooling and a shmexy case!