I've got 6950s and 6970s.
RBE warns on certain changes that the native drivers may stop recognizing the card.. and someone else mentioned after a certain RBE edit even aticonfig wouldn't find the cards.
Are you using a signed 'rbe' excerpt from another card (~1kb) to get around that?
I cannot for certain say for those, but I am running 6870's, with RBE modified bioses. The newest version, 1.28, supported overvolting them, which is pretty useful. The doom and gloom forum posts about the cards not being recognized in windows and BSOD's seem to be limited to the windows world. I used "method 2" , aka the no checksum, direct write, screw it lets see if it works method, to increase my overdrive limits to 1200 MHZ on a couple of cards so far, and linux has been happily mining away now for days on them as I test for stable clocks.
Note, I changed the GPU register, modified the fan curve, and used method 2 to increase the max overdrive, but I didn't mss with any powerplay states, so I don't know if that will work or not.
Underclocking memory would likely mean modifying the powerplay states, and I have been too lazy to do that thus far.
There was a post I made about this on the techpowerup forums, but it is gone to the nether since apparently putting a hosting company in your .sig is a bannable offence. Not that I am bitter.