I ran 560Ti 2GB SLI for a short while. The FPS was OMGWTF amazing for a while, but I noticed that vs. a single card (7970) there was just that tiny bit of choppiness/jerkiness. Add to the fact the extra fan noise and that it wasn't perfect 2x scaling like was mentioned above, I made the choice to go back to single GPU.
Plus, the premium on the 3GB RAM version is ridiculous, and so would be going back to 1.5. I say ditch the 580. =D