Have had enough time to get a general idea of speed on certain frequencies. This is with a Gold EVGA SuperNova, if you have Platinum psu you likely will be a little less. And it's on 110 so again if on 240 a little more more efficiency.
It is pretty close on efficiency no matter what freq you choose. Above is not perfect but a very good general idea from a few tests. But you are able to lower frequency if for some reason you wanted to run on a smaller PSU. But as of now running it at default of 500 on frequency seems like best idea.
On underclock likely it is going to be input voltage that makes it possible.