I have a 7770, and noticed something similar. When using CGMiner, I can't put the mem speed more than 150 below the core speed (which is what other people have found as well). However, using AB and the UnofficialOverclockingMode=2 tweak, I'm able to set the mem speed as low as I want. I ran it at 300, confirmed through CGMiner and GPU-z.
Im able to set core and memclocks, core and memvoltage PT and all with afterburner, but as soon as I enable overclockmode 2 It blanks out the sliders for core and mem and no longer works.
Tried many drivers to no avail. Next step is to try some beta drivers.
kind regards