No it won't make any significant difference to go from x16-x8 mainly because graphics cards can't fully utilize x16 lanes just yet. Take into account that pcie gen 3 is new and does twice the performance of pcie gen 2, so pcie gen 3 at 8x is essentially a pci gen 2 x16 performance. You will see around 3% performance diff between x8 an x16. In sli this is reduced even further.
Hope this helps. Cheers.
Hope this helps. Cheers.