I have a few MSI 56 cards that have crap Hynix mem in them and they all work with edited registries lowering all P-states. I have run memory voltage down to 850 or something and they ran, it was the clock speed mine were finicky about. They don't seem to mind any amount of undervolting BUT they will not handle the same memory clock speeds as the 64's or the one 56 I have with samsung memory. I had a lot of trouble getting my stuff fully stable in the beggining since I kept reading in so many places that the 56's would run 1000 or even 1050 on the memory clock once the BIOS was flashed. I have a few cards that won't even run at 1000 on the mem clock. I currently have all of mine down to 960 so I don't have to worry which card is which and I get good enough hash rates at that mem clock. 100% stable for days on end. I did have to pick the right BIOS carefully... For MSI the "latest" 64 BIOS did not work at all. MSI brand showed 3 options for air cooled 64 cards and only one of them worked, it was a total nightmare figuring all that out! Though  I bricked a few cards one night, such a nightmare.