One additional comment, the user with the XPS 9370 was able to get the RX 580
working as an eGPU flawlessly in Windows 10.  This lends some credibility to
the theory that it might not actually be a BIOS issue - unless the BIOS bug is
worked around in Windows 10 drivers.  Please see here for additional
information:

https://forum.manjaro.org/t/rx-580-in-a-thunderbolt-egpu-dock/58210/30

I'm planning on purchasing an Vega 56 or 64 in the near future so I can
continue to attempt to troubleshoot the issue.

Rob