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
Maybe I'm wrong but those cards do not require dummy plugs. They either work or they dont, not represent a lower hashrate if you don't use.
You can't OC some 4XX/5XX AMD cards without dummy plugs. I think it has something to do with (ULPS) Ultra Low Power State bug, where your computer thinks the GPUs aren't being used so it downclocks them.
I have 560's 570s and 580's and I do not need a dummy plug for any of them
I don't need dummy plugs for mine either but some people do.
You can mine without dummy plug, but to see temps/fans in CM you need dummy plug on 1 card, I just put one on each primary card, plugs are like $7 on Amazon. Also makes Teamviewer not suck.