OK, I've sorted this.

Copied /boot/efi/EFI/GRUB/grubx64.efi to /boot/efi/EFI/BOOT/BOOTX64.efi

THAT did the trick. Noticed that difference between this new install and the install on the Aorus (which I had done some time ago), and confirmed it with a Slackware page about UEFI GRUB installation:
http://docs.slackware.com/howtos:slackware_admin:set_up_grub_as_boot_loader_on_uefi_based_hardware

Additionally, according to this doc, some versions/implementations of UEFI are case-sensitive (and look for bootx64.efi (note the lower case)):
https://help.ubuntu.com/community/UEFIBooting

What a pain in the ass!
:oP