Same issue here.  The lowest I can downclock it to is 850.  I think it has something to do with the voltages.  If you look at the voltages, it the mem clock speed is 300 at 1, and then goes to like 950 at 1.17.  So I think it is just a limitation of the card due to the voltage changes.  850 is the lowest I can go at 1.17.