From what I know, 2 Gtx 560 Ti's run faster than a GTX 580 in most cases. It also depends on what resolution you're planning to run at. 1080p or something is better with the GTX 560 Ti's but for insanely high resolution gaming, a GTX 580 would be better.
Additionally, I would suggest getting the GTX 580 over getting 2 GTX 560 Ti's unless you plan on buying one NOW and then ANOTHER later, as how I plan on doing my build :P
Now to the main answer of the question:
- Dual 8X mode and Dual 16X mode usually have un-noticeable differences from what I have been reading online. Usually they offer similar performance.
Hope that helped.