The purpose of underclocking the memory clock is so that you can overclock the core clock and not fry the card. The combination of lowering one while raising the other will give you an overall higher hash rate while keeping temperature the same or not much higher (and in some cases, slightly lower). Just be careful and only bump the core clock up in small amounts... a dead gpu doesn't hash
I thought people lowered the memory clock because it didn't affect their hash rate while simultaneously reducing heat and power consumption.
I don't see any core overclock pushing my hash rate from 160 Mhash/s over 230 Mhash/s... Stock core is 775, so 925 is already a fairly big overclock, and at 925/300 I get 160 Mhash/s.