I saw the same results with a 7950 with OS X and a 6870 with Windows 8.1. The best way to get this fixed is to Enter a bug at apple (see my post above). It took less than a day to hear back on broken HDMI performance on OS X. With further research, it turns out that Apple is limiting HDMI bandwidth to a 1.0 specification of 165Hz. HDMI was updated to specification 1.3 in 2006, raising the limit to 330Hz. As long as trey can figure out where to change 165 to 330, it should be an easy fix. The same goes for your bug.
While most assume Apple knows about these issues, their quick response and request for more info tells me otherwise.
Unfortunately I can not send a report. It will not let me log in with my AppleID. I would assume a developer account is required?
- Garrett