Undervolting memory may only be supported through BIOS mods...
it would be interesting to know about power consumption at lower mem clocks. If you REALLY want to know, I can ask him, provided I give him enough $$. I would need something like 15~20$ I guess. If enough people want this and contribute, I can ask him ho much he wants, perhaps he would do the same for a 5870 and 5870 if he still has those too.
I've checked all of my current BIOS (5850 / 5870) and unfortunately I can't access the VTT registers in any of them.
I do want to know, but I'm patient and not looking to purchase any 6990s in the very near future. I'm sure there will be miners that use them and are willing to provide their info. While it may not be as accurate as W1zzard's, it will be interesting none-the-less.