This will be the 3rd time I've tried posting this here. I'm on Linux and used CoreCTRL to undervolt my new 6800XT (I had to use 3rd party software because I'm using the drivers baked into the kernel and those don't have any of the fancy things the Windows drivers have). I used Unigine Superposition in 4K mode as a stress test. Here's a spreadsheet showing my various undervolts and results:
Undervolting AMD 6800XT with CoreCTRL on Fedora and Unigine Superposition (4K)
This is great! I've been looking for something like this for my 6800XT. I managed to run something stable around -120 on mine but it has recently been less and less consistent.
Unfortunately, I've once again moved away from Linux and have completely forgotten what I did with CoreCTRL. I *THINK* I left those min/max MHz sliders at their defaults and just worked with pure undervolting. Sorry.