Juan Jose (the Senior Technical Marketing Specialist at ASUS) says this about Internal PLL Overvoltage:
"This option is specifically designed for D2/Retail Parts and allowing for higher multi overclocks especially when at or exceeding 48x. Additionally, this option can be used if you do not have it enabled and generally cannot boot with a higher multiplier. It is advised to try this option after you have tried all other standard options. This option is not for increasing stability of a OC that can already post and boot into an OS."
So he's saying that while it's designed to allow 48x or higher, it's not be necessary to have it enabled if the system can boot with the higher multi.