Read up an article somewhere or was it just a marketing advert that exisiting cards gain some performance boost on the newer bus, true or false?
Likely false. The X79 motherboards are marketed as supporting PCI 3.0, but require a BIOS update at a later date to enable it. Considering the hit going from x16 to x8 on a PCIE 2.0 is about 2-5%, I don't think PCI 3.0 will bring much from a PCIE 2.0 x16 slot. I guess it all depends on how much more powerful new cards will be.
Considering a PCIE 2.0 x16 slot can handle a 6990, I can't see the 28nm GPUs needing anything more than that, unless we are talking the dual-gpu cards that will be coming, then maybe.