Yes, you do it in BIOS under the M.I.T. heading and it is the CPU Voltage Control. There are no hard and fast rules for how much you can pull back the voltage before the system becomes unstable but, for example, I have pulled my 1090T back by 0.050 V.
It is really case of suck it and see and if the system does fall over, because you have pulled it back too much, you can always reset it a little higher. I would suggest starting with 0.025V first and if it is OK try pulling back a little more.
As for performance, when it is set correctly it won't affect it and all you will see are lower temps on the CPU.
Sorry, I removed my post about recommendations for stress testing applications since i figured I might as well use good ol' Google instead of bothering you about it. But you were too quick .
No problem.