Asus p6x58d-e 16x8x8x.

Greetings,
After several hours of searching I couldn't find the option in my bios to set my 3rd pcie lane to 8x instead of 4x.
So basicly I'd be going 16x 8x 8x instead of my current 16x 16x 4x.

Any help would be nice.

Your board shouldn't run x16/x16/x4, it's either x16/x16/x1 or x16/x8/x8. You can force the link width in the NB config in the BIOS.