My 14.5TH s9i by default running on 14.5TH and consume 1330W/h at default frequency => 675Mhz and 8.6V
I get 14.5TH by changing voltage to 8.3 with power consumption 1230W/h at default frequency => 675Mhz
It means that i save 9% of energy on 8.3 volt. at lower voltage the hashboards needs more time to achieve their hashrates .
Actually it is good if they manage to achieve their ideal hashrates after a while. You shouldn't rush it. And remember to treat each hashboard differently, you will find some can take voltages that others can't. They are not identical.