since you gave a cpu/gpu capable of pci-e 3.0
x8 and x16 would have absolutely no difference.
Even if it did, it wouldn't be more than 1% for gaming.
but in your case, there is no difference between a pci express 2.0 x16 and pci express 3.0 x8
gtx760 isn't that bandwidth hungry card anyway. Also for reference, I have my gtx670 hooked to a x8 slot to make room for extra fans. No performance deterioration whatsoever in every benchmarks