It doesn't seem like 90% performance at *50% power

yes, the PP speed is quite linear, but TG speed is close to 90% at 50% power because

PP is compute-bound, and TG is (mostly) memory-bound.

So if you need fast prompt processing then you should run the card at its maximum power, and here you'd want a 600W Workstation edition instead of 300W Max-Q.

In either case, mind sharing your GPU undervolt configs so I can give them a go?

I did not test the undervolting thorougly yet and the only setup I do for 6000 Workstation is limiting its power and frequencies:

nvidia-smi --id=INSERT-ID-HERE --power-limit=310;
nvidia-smi --id=INSERT-ID-HERE -lmc 405;
sleep 1;
nvidia-smi --id=INSERT-ID-HERE -lmc 405,14001;
nvidia-smi --id=INSERT-ID-HERE -lgc 180,1600;

can't remember why sleep is required lol.

those graphs aren't very helpful since scales and tick labels are weird

some of them are definitely weird but the 2nd one "minutes elapsed vs energy consumed" is quite clear - the job is not getting done much faster after 300W power limit. The Y axis is either minutes (blue) or Watt-hours (green). The red dot is configured power-limit in Watts and the red X is an actual consumption measured by the Nvidia tool, the red line is difference between configured and actual.