Mining doesn't need a fast memory clock. I run 5970s at 150 Mhz.  The ban news is without a bios flash (risky) 6000 series only allow mem clock that is 150 Mhz lower than core.  So if you are running 900 Mhz core you can try 750 Mhz memclock.   Remember if you later raise the core clock you need to raise the memclock.   Nothing "bad" will happen if you try to set it too low.  The card will simply ignore the request and run it at stock.

Looks like the 7000 series don't have this "glitch". Only the 6000 series does.