Hi there, I am currently running a RTX 2080ti. It does run at full speeds.
However, you need to know that it only works if you install Win 10 in UEFI mode.
I couldn't boot the system with Win 10 in BIOS mode.
Edit: If you read through the bootrom thread, using Win 10 in UEFI mode can cause serious boot rom corruption which could actually brick your mac pro. This is why I wanted to go back to BIOS mode... and that's when I figured out that the 2080ti does not work in that case