I'm not sure that that is true. I looked at the cgminer sources for the DM and it definitely looks like that version of miner software handles frequencies up to 850. Looks like if you specify above that, it just gets capped to 850.

Maybe this is just the Linux version? I have no idea what the Windows version supports.

Earlier in this thread others have mentioned using 850 and gotten modest returns. For example, Poopypants said he was getting 72-75kh/s at 850. I tried using it too at one point, but backed off while trying to get my rig stable.

Now that my rig is stable, it would be nice to squeeze a little extra out of it. I might try it again...