If I understand things right, once I plug in the dummy, the AMD drivers will load properly, and the 570s should get into the 28MH territory.
Correct
I have two rigs not connected to any monitor one with 3 560s and 1 570, the other with 2 580s and 2 1060's , I have no dummy plugs and I get full hashrates out of them. If I connect a monitor and check them I can go through their history and see consistent proper hashrates for all the cards.