I can get 295 s/s out of 390 without bios mod but there is no reason to push it to the limits when you can do 240 with -50% on power consumption.
ok, add new info
480*5 900h/s 1145/2100 (bios mod starp 1750) -60mv 650w from wall
650/900= 0.72 best result)
and
i get 285 h/s on sapphire 390 1100/1500 -50mv but i dont know power consumption.