Overclocking In SLI
Overclocking in SLI may yield a 15-20% performance boost, or more. For reference, my own personal results in 3DMark Fire Strike are available here. I'm working with an EVGA Superclocked version of the 980s, so I'm starting from an already-high 1266MHz base clock rate and 1367MHz GPU Boost rating.
Overclocking cards in SLI is not always straightforward, however. In particular, we've noticed that the bottom card (that is, the one in the lower PCIe slot), regardless of voltage input settings, ends up operating at a lower voltage than the top one. That is undesirable, as it appears to limit the overclocking potential of both cards, since they can only go as fast as the lower card, with its limited voltage increase, will allow.
When asked about the voltage inconsistency, Nvidia answered, "We don’t run each chip at the exact same voltage. There’s variation from chip to chip." For 99% of users, this shouldn't be an issue they need to worry about. If the voltage inconsistency really bothers you though, there is a workaround: unlink the card settings in a tool like PrecisionX and set the card with the lower voltage at a higher core clock offset than the other. This won't lead to core clock rate differences; the frequencies will still be locked at the same number. But it will raise the voltage of the card to match. Not elegant, but it works!