i'm running 12.3 with latest sdk and I oc at 950/1200 (bios mod) for gaming (watercooled, never above 37c) and if I downclock the memory to 300, I save around 50-60watts (which I honestly can't say I care about since I got a fixed rent which includes electricity).  at 1200mhz memory, I get between 770-820 mh/s on the pool management software, while I don't know how much lower I'd get at 300mhz memory.  That's on CGMiner 2.3.1-2 and phatk.