Hi,
it's not. Don't use cgmiers gpu-vddc. Use Sapphire Trixx SW but you have to setup voltage on every PC restart.. To avoid setup after each restart, lower the voltage, save bios thru GPU-Z and flash it to your GPU (at your own risk).
For 1080MHz use 1089mV, for 1120MHz at lest 1130mV. You'll also save a lot of energy (up to 60W/card) of which you can donate me some
The vapor-x is more sensitive to temperatures in some cases, but if set properly it can give you 780kH/s.