ETH is memory bound, core clock effect is minimal on Vegas.  Instead of setting P7, simply disable all GPU power states above P0 in OverdriveNTool.  Also, your memory clock can go up to 1107  for a 64 and you should be able to use a voltage setting between 806-831mv.  Above 1107, your voltage requirement jumps significantly to around 900mv (due to SOC clock scaling to 1200MHz.)

For a 56, your voltages and core clock would be the same, but your memory clock will have a fuzzier limit than the 64 - somewhere between 900-950MHz most likely.