Try 980/2200 MHz at 0.9V (or less if possible). Should drop power usage 10+Watts per card (reported by GPU-z) and the mining speed is still neat 26MH/s
Also you're currently only pulling 900W on a very high quality power supply anyways. 91% efficiency as well if Corsair link is to be trusted. Nothing to worry about here. 1200W is a bit of a waste for 6x480's.