Did some more testing, this time with a Nvidia card to see if the issues
occurred there and with a different DE.

I now think the stuttering I'm experiencing is purely a Gnome issue. I gave KDE
a try and 75 Hz is completely smooth, just like in Windows. I also tried the
1080 Ti in Gnome and it had the same stuttering my WX7100 and Vega 64 have.

The artifacts definitely seem to be AMDGPU related though. The 1080 Ti doesn't
artifact at all when running at 75 Hz.