Not all the 3930k's actually support PCI-E 3.0. Many failed to meet the spec and hence NVidia disabled the 3.0 support on the platform. Forcing it could cause instability and from what I have seen brings no performance benefits.
i7 3930k @4.4, 2xMSI GTX 680, 16GB Corsair 2133 RAM, Crucial m4 500GB, Soundblaster Z
Custom watercooled by 2x MCR 320 and 1 MCR 480
Zowie Evo CL EC2, Corsair K70, Asus Rog Swift PG278Q