Just download & use the Tonga Bios editor to edit the voltages and frequencies.  You can also look up nerdralph's thread on how to modify the memory timings to increase hash rates further.  For that you will need to use a hex editor after which you will again need to open the bios file in Tonga BIOS editor to fix the checksum.   My Sapphire 380X does 23.5 MH/s mining ETH/ETC and it is running at 1000/1650 core/mem.