Assuming IB comes with a PCIe 3.0 controller, if you dropped it in any current (P67) high end motherboard you'd be limited to PCIe 2.0 speeds right? Since most boards come equipped with PCIe 2.0 speced(?) multiplexing hardware (like ASMT lane switches) that allows either a x16/x0 or x8/x8 split.

Ironically entry level boards where all CPU lanes are hardwired to a slot don't have this limitation and could take full advantage of the 3.0 controller (provided cougar point gets validated).