Im having this same issue on my 7950.
Memory will not underclock more than -150 of core.
Tried cgminers mem-diff
Tried AB and trixx. Nothing will underclock the memory below the -150 threshold.
In fact if i do try, it resets the memory speed to 800mhz.
[update] : Interesting, from another thread here :
https://bitcointalk.org/index.php?topic=93716.msg1035879#msg1035879 (same issue, using ati 7770)
Well I finally found that going back to 12.3 fixed it or "allowed" me to use a combo of afterburner & barely clocked to sneak in the memory settings to 300mhz.
Im using the 12.6 drivers, perhaps a different version will allow the greater memdiff, Ill report back if successful.
kind regards