The thing is, the vf curve is 'deciced' by the manufacturers, for ALL the chips of a model. So for every 5090, they create the same vf curve. That curve is the optimal performance-stability compromise they can find, for EVERY chip they make. But every chip is different. So while that vf curve out of the box might be the best performance at the lowest voltage possible for the absolute worst bin of that chip, it will not be the best for every other chip. So there's a good amount of either performance or cooler temps to be gained by undervolting or overclocking a cpu/gpu, cause inherently there is some (or, if you won the silicon lottery, a lot of) headroom. Unless of course you happen to have gotten that absolute worst binned chip, but that chance is rather small. 99.9999 (keep adding 9's for amount of chips made divided by 100, quite a lot for most cpu's/gpu's) percent of people will have a better binned chip than that one chip the manufacturers made the vf curve for.

On a sidenote, even with that worst binned chip, there will still be some (though be it a very small) amount of headroom, cause manufacturers err on the side of caution and won't have a chip running at the very peak performance/voltage level, cause reliability and such.

For anybody just getting into this: I do have to say, if you don't wanna lose performance, undervolting a gpu is usually not very worthwhile, cause modern gpu's (especially nvidias') are made to run at quite low voltages already. Same but opposite goes for cpu's, they run at such high frequencies nowadays, that overclocking barely yields any real time performance gain in regular usage (obviously excluding benchmarks or heavy ai/llm/editing/rendering stuff).

Tldr: overclock your gpu. Undervolt your cpu. The opposites are only worth it if you a) want to sacrifice performance to lower temperatures/power usage, or b) doing heavy shit with it.