Get yourself a D-tek set
It's the shaderclock that holds overclocks down, and the shaderclock is very sensitive to heat, so good cooling is more important than voltage.
At standard 1.19v i ran my cards 774/1593/1296
At 1.5v i ran my cards 828/1701/1296, but after 5 minutes artifacts apeared, shadercore getting too hot.
So back to 1647 on the shader and thanks to that, max core clock to set is limited at 810.
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