A few interesting observations on Thaiti GPUs. First, it looks like similar to Fermi, Thaiti GPUs are binned and there are 4 types of Tahiti GPUs with different ASIC leakage level. So depending on it there are 4 possible default 3D voltages: 1.17500V, 1.11250V, 1.05000V and 1.02500V.
Currently all voltage control tools use hardcoded default voltage (1.175V) for 7970, so pressing "Reset"/"Defaults" buttons in any voltage control tool will put any GPU to 1.175V regardless of leakage level.
It looks like I have located the fuses identifying GPU leakage level, but I need more statistics on it. So if you have 7970 please run MSI Afterburner with the following command line switch:
MSIAfterburner.exe /rr174
Then post the result here, and don't forget to mention your default voltage as well.