Dominate? No.
It's difficult to calculate the differences in performance for *any* two gpus running *any* game at x16-x16 vs x16-x8 vs x8-x8. The question is whether *this* game would gain (noticable) fps from *this* pair of gpus if I went from, say x8-x8 to x16-x16.
The best analysis I've seen to date is this one (I've linked you to the SLI-summary page, but you may want to read other sections and the conclusion):
http://www.tomshardware.com/reviews/pcie-geforce-gtx-480-x16-x8-x4,2696-16.html
If you are considering SLI, I think its fair to say the following:
1) Avoid using an X4 slot. You *will* most likely see a performance hit.
2) x16-x16 is theoretically the best (duh).
3) x16-x8 *may* show a few percent performance loss while gaiming vs x16-x16, but most likely not.
4) x8-x8 will lose you a few more percent of SLI scaling in general, and I'd personally avoid it if contemplating two very high end cards (eg 2x580).
An x16-x8 mobo - in my opinion - is a very reasonable choice for a rig gaming at 1920 and wanting to preserve the option of SLI, or when SLIing mid-range cards.
At 2560 or higher resolution, I'd probably want x16-x16 because today's games begin to challenge today's best gpus at those resolutions. And I wouldn't want to give up even a few percent of performance.