You can't change the core voltage on your mobo unless you do a volt mod to your mobo. Do a search in this forum, there are extensive posts on this subject.
As for the multi, I don't think you can change it in the bios unless cheepo found a way to come out w/ a bios that allows multi adjustments.
The wire trick is commonly used by XP2100 tbred owners who don't have access to 12.5x and lower multis so they can access the lower multis in place of the 13x and highers multis.