You may can squeeze out a few GH/s by setting it, but there is some testing needed, as each miner has it's own chracteristic
Setting diff has zero effect on performance except for completely broken hardware designs. That said, the first S4 firmwares were that sort of broken (long since fixed now), and just about every dragon miner was too, but everything else was NOT, and setting diff there has no effect on anything at all so there is no squeezing anything out of anything for the vast majority of hardware.