BootROM has nothing to do with driver support.
Any GPU from NVIDIA newer than Kepler has zero driver support with Mojave until NVIDIA releases the web drivers.
If someone got a 1080 showing screens with Mojave, it's a Mac EFI flashed GPU and is running the un-accelerated default EFI drivers, so it's unuseful to work as is so sluggish to do anything besides showing the screen.
140.0.0.0.0 has support for PCIe booting from NVMe and AHCI, your AHCI PCIe blade will continue to work as always worked before, no changes. We expect that Apple will release 140.0.0.0.0 with 10.14.1 in the next week or so.
Don't do the flash from the USB installer, do from macOS. Use a Mac EFI GPU to flash until MP51.0089.B00.
If you are having problems with macOS flashing, probably you are using some non standard macOS install and the installer can't bless your drive. Get a empty drive, install it onto the SATA bay, install 10.13.6 into it, update the firmware to MP51.0089.B00 then to 138.0.0.0.0.