I have multiple Sandy Bridge/Sandy Bridge-E GPU/CUDA development machines and while I could get both GeForce GTX680 and TITAN to work in PCIe 3.0 mode (using the NVreg_EnablePCIeGen3 driver option, available since v313.18), I failed to achieve this with a K20c.
Searching around on the internet seemed to confirm that others also are also having trouble, but I found no clear confirmation whether the Tesla K20-s are limited in the driver or in their VBIOS to PCIe 2.0.
So my question is: are the Tesla K20-s effectively crippled to PCIe 2.0 in their BIOS? How about the K10 and Quadro-s?
Note that I use Linux, and I've tried a few of the recent 313-series drivers.