Just a quick FYI. My Asrock fatality pro (2011) mobo, allows you to mix pcie 3.0 and pcie 2.0, you can configure any of the 3 pcie16 slots to run version 2 or 3. I might try it tomorrow.
any 3.0 slot will run 2.0 cards
not sure why you would need to change anything in the bios