My computer has 4 PCI Express slots:
PCIe_1 and PCIe_3 is an x16
PCIe_2 and PCIe_4 is an x8
I put my card in PCIe_1 (x16) and ran 3Dmark test then put my card in PCIe_3 (x8) and ran the same 3Dmark test again.
However, I don't really see any major differences in the score, I expected x16 to perform much better.