Overvolted 390 can't do better than that? Set your fan speed to 100%.
Can be up to +100 mV, however at 1175 MHz for the GPU white lines appear at the monitor (max stable overclock is 1150 MHz at +20 power limit and 50 mV Core Voltage).That's why I use the 1140 MHz for the GPU and 0 power limit (can be a minimum Core Voltage at +6 mV).