Note: On my Vega 64, highest stable HBM overclock on the reference cooler was 1020MHz.

The memory voltage label in Wattman is wrong, it does not undervolt HBM, it is more like a core "voltage floor", or power budget to be shared among core and HBM.

Your memory voltage tells the GPU what voltage it will NEVER GO BELOW. So this should always be equal or lower to your LOWEST voltage in P6 or P7.

As a consequence of the above point, it is useless setting your P6 or P7 to a voltage lower than memory voltage. It wont ever use it as memory voltage is the minimum.