Fun times: I'm getting 1400 kh/s on a Radeon 7970 with opensource Linux drivers. Default clocks of 950/1450 MHz and I estimate the power draw at 150 W max.
Are you able to set clocks with that?
Yea, but not any higher so far. The "high" profile gives the same frequencies as "default". What's worse, all non-default profiles go to "low" speeds when the monitor is off.
For details see
https://wiki.gentoo.org/wiki/Radeon#Power_management and in this case, dynamic power management must be disabled with the kernel option radeon.dpm=0. The dynamic setting doesn't have the monitor (DPMS) issue.
For comparison, using fglrx gives 875 kh/s at 1100/350 MHz, about the same power judging from similar temperatures.