Changing the frequency alone won't significantly increase the efficiency. You have to also reduce the chip voltages through the pencil mod or adding resistors (using voltage setting available with some firmware does nothing). I reduced by adding 8 resistors to each hash board of 4 S3s. I set each to the frequency that was the most efficient. The slowest hashes at 250 and the fastest at 390 Gh/s. I'm now averaging 0.62 J/Gh, which is still profitable.