While I dont have any risers laying around my motherboard has PCI-E 2.0 x16 slot running at x2 and a heavily overclocked GTX 780.
Here is my config for my GTX 780: cudaminer.exe -q -i 0 -m 0 -H 2 -l T12x24
PCI-E 3.0 x16: ~715Khash/s  with a bus usage of ~13%
PCI-E 2.0 x16(x2): ~685Khash/s with a bus usage of ~40-50% (I forgot the exact number)

If you have any more questions feel free to ask.