Moving from 1.7Vnb to 1.825Vnb on the Gigabyte X48T-DQ6 ( "X48" chipset ) allowed me to run SuperPi 32M at PL6 when with 1.7Vnb best I could do was 8.
The same thing ( more NB voltage helps with PL ) applies to X38/P35/P45.
As long as you can keep the chipset cool, 1.8V-1.85V will be fine.
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