OK, so I just NOW got my shipment of the Quadro K600 cards!
Got 7 cards shipped for about $120 bucks, including tax.
CONFIRMED (of course) that they do indeed fully support Metal / Mojave (Catalina? Big Sur?).
No Apple logo screen, as known, but my Mojave SSD boots from chime to desktop in about 20 seconds!
Since I am running a Mac Pro tower here, I'm going to add this for anyone (like myself) who arrives here.
Easy, proper install of Mojave WITH the proper firmware update on a Mac Pro 4,1 or 5,1 goes like this;
I used an existing High Sierra install / drive, and then downloaded Mojave with a Metal capable card in the tower.
My intent is to install Mojave on a separate empty drive in the Mac Pro, which makes this super easy.
Upon starting the Mojave install from WITHIN High Sierra, you'll get the message regarding a Firmware update required.
At that point, you will (as per instructions there) shut down the Mac Pro and then restart WHILE holding down the power button.
You will see NO boot screen, of course, but CONTINUE to hold in the power button. You will hear the chime, then a low tone, and the power button will begin flashing.
CONTINUE HOLDING THE POWER BUTTON. If your DVD-ROM drive ejects, then, at that point you can finally release the power button.
Your Mac Pro will re-start itself after the firmware update and you will now be able to install Mojave (again, while in High Sierra) to the desired drive.
I know that there are other ways to install Mojave alone, but I prefer not to try doing this "blind" with no boot screen. Again, this is for Mac Pro owners.
THANKS again to trs96 for this GREAT informative post regarding the Quadro K600 Metal support!
P.S. Even though I mention installing Mojave / firmware upgrade on a Mac Pro 4,1 . . . the 4,1 firmware must FIRST be upgraded to 5,1 firmware. Plenty of instructions for this elsewhere.