Hmmm, this is really interesting data. First, I do think your GPU is performing well. At least in the Farcry benchmark your not seeing sub 40 fps you mentioned in your original post. What is a little strange however, is that you didn't see any gains at all by undervolting and raising the fan curve. That implies that some other component is bottlenecking the system as opposed to just the GPU.
The only thing I can see that is different, is that I am still running the last "Summit Ridge" AGESA UEFI on my motherboard. You mentioned you are running the latest BIOS, which would include "pinnacle ridge" support (aka, 2700X, 2700 etc.)
Since you aren't running a pinnacle ridge CPU, there is no reason to update to the latest UEFI, try running the last "Summit Ridge" update for your board. Looks like that is 7A33v36.
Out of curiosity, did you ever get this issue resolved by the new psu? I recently picked up a Vega 64 and it crashes to a strange pixelated rgb looking screen every time I try other put load on it, I'm wondering if it is because I am using a daisy chain power cord for it. Let me know thanks