Assuming your sig is correct and you have a sandybridge processor you will not be able to get PCI-e 3.0 BW as the pci-e 3.0 lanes originate from the CPU . if you dont have an IB CPU you really do not have PCI-e 3.0 lanes, even if your board is gen 3 certified. You would need to pair it to a IB processor to get PCI-e 3.0 bandwidth
This is also listed on the ur board's web page:
read below "PCI Express® 3.0"
* Actual PCIe 3.0 speed varies with the installed CPU type.
The z77 PCH diagram (link below) elucidates this point. Look at where the PCI-e 3.0 lanes originate from:
Older (<PCH 7), that have certification for gen3 only have lanes that are 'sturdy' enough (electrically) to carry over the PCI-e 3.0 bandwidth.