I think it's more complex than that because there's another factor to take into account - power draw.
I discovered the potential of undervolting with the Radeon 7950 I had some years ago. I got a very large increase in performance at the same clocks by undervolting. Temps and noise were reduced, but that wasn't what caused the increase in performance. Monitoring showed that the card was power throttling, not thermal throttling. The card was overvolted out of the box, by a surprising amount. IIRC (it was a while back - my memory isn't certain) I was able to drop it by ~200mV.
My current card (a 1070 Ti) is still limited by power draw despite apparently being undervolted out of the box (max GPU voltage is 90mV lower than the spec for that GPU). Even when I increased the power limit by 30%, it was still limited by power draw. I tried undervolting it further, but it was unstable with lower voltage. Maybe I could shave a few mV off, but that's neither here nor there. I run the card at stock everything anyway - any fiddling with settings is purely temporary for benchmarking as a sport.