OWC Aura SSD Firmware Update from Mac Recovery Terminal
I'm responding to this post late, and my contribution is a workaround for Aura SSD install/upgrade to High Sierra on a Power Mac, late 2013.


Install High Sierra on your OEM SSD.
Shut down, remove power cable.
Move the OEM SSD to the Envoy enclosure.
Install the Aura SSD into your computer.
Connect your OEM SSD (in the Envoy enclosure) to the USB port of your Mac.
Power up, zapping the p-ram (hold down Option,Command,P,R)
The computer should boot from the OEM SSD (in the Envoy enclosure). If not, restart and hold down the Option Key. The computer will show startup disk options. Choose the OEM SSD.
Once you have booted from the OEM SSD, start Disk Utility.
From the menu, choose "Restore". Select the OEM SSD as the source drive and the Aura SSD as the target drive. This will clone your OEM SSD to the Aura SSD.
Power off the machine, disconnect power cable, and disconnect the USB from the OEM SSD.
Power on, and the machine will boot from the new internal Aura SSD. If not, zap the p-ram again. Also, if you're on a Mac Pro, do not use wireless Bluetooth keyboard/mouse throughout the process, use wired ones.
This worked for me, hope it'll work for you.