I think most motherboards if not all show the GT/s to be 2.5 In BIOS and actually most of the time if there is no load on the GPU the GT/s performs at the 1.1 GT/s 2.5 could be some form of power feature to save power, but then again i could be wrong.
Anyway here try this and report back, so that we know if yours is truly stack on the 2.5 GT/s 1.1 PCI-E X16 Speed
My screen shots even thought they are for GTX 770
Start the render test and check if it moves from 1.1(2.5GT/s) to 3.0 (8GT/s)
If it's still stack at the 1.1(2.5GT/s) then we may have a problem and Intel will have to produce a BIOS to fix it