PCI Express 3.0 Has Zero Performance Incentive for Radeon HD 7970: Tests
GPU maker AMD wanted to be the first to be out there with a GPU that's compliant with this interface, and so one thing led to another, and VR-Zone got to set up a test-bed using Core i7 "Sandy Bridge-E", ASUS Rampage IV Extreme (which allows users to change PCI-Express standard mode in the BIOS setup program, by forcing Gen 2 or Gen 1 mode), and an HD 7970, to see if running the GPU on PCIe 2.0 and PCIe 3.0 modes made any worthwhile difference. The results are in: zero, nada, zilch, sunna (zero in my language).
In its comparison, VR-Zone put the GPU through 3DMark 11 (a DirectX 11 graphics benchmark) and ComputeMark (a GPU compute shader benchmark that heavily loads system bus). The performance figures between the two were agonizingly insignificant. 3DMark 11 and ComputeMark are tell-tale tests of whether the GPU (and with it, its system interface) is at least getting loaded enough. You would much rather spend the money you saved to upgrade your current, perfectly-functional LGA1155 motherboard to an "ooh-Gen3" one, on a memory upgrade, before DRAM prices rebound.
One area, however, where Gen 3.0 could have a performance incentive, could be with future Ivy Bridge LGA1155 platforms, where to run 2-way CrossFire, the single x16 link from the CPU is split into two PCI Express 3.0 x8 links. Those numbers could be interesting.