performance loss running 2x 780s on my asus z77 sabertooth (8x/16x = 2x sli being 8x/8x)

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.

no man your good and that's actually the 1155 mobo standard as only the few more expensive boards
on 1155 run x16/x16..
but again your fine and will have NO performance issues.
x16/x16 is a few frames better at higher resolutions like 5760 x 1080p and up
but even then it's perfectly fine @ x8/x8.
enjoy.

cheers.