Short answer; Yes GTX 780s SLI can out perform single GTX 980.
 
Hope this helps you decide.  I found the following information:
My simple Google search (SLI GTX 780 Classified vs GTX 980) turned up:
 
http://www.tomshardware.com/answers/id-2312797/gtx780-classified-sli-gtx-980.html
 
Also found real world test results:
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_970_SLI/22.html
 
excerpt:Value and Conclusion page 22
"Across the competitive landscape, the GTX 970 SLI demolishes every NVIDIA single-GPU graphics card we have with us, be it the GTX 780 Ti or the GTX TITAN. The dual-GPU Radeon HD 7990, which held onto its lead over an impressive streak, is as much as 29 percent behind in performance. AMD's current flagship, the Radeon R9 295X2, however, is 6 percent faster. It's about $330 costlier, and even if we were to assume that the GTX 970 SLI configuration has double the power draw of a single-card, the duo would still end up with less power draw than AMD's offering. If you're feeling adventurous, you could spend the $330 saved on a third GTX 970. With most $150-ish Intel Z87 and Z97 chipset motherboards shipping with a third PCI-Express gen 3.0 slot that works at x4 when three slots are populated (bandwidth comparable to PCI-Express 1.1 x16), a third GTX 970 could be an audacious alternative to spending $999 on a single R9 295X2. The GTX TITAN-Z isn't budging from its $2,999 price-point, and so it's no less than one parsec away from our thoughts."
 
You do need to consider this; also page 22:
 
As with every multi-GPU setup ever made, the GTX 970 SLI has its shortcomings because of driver support. Games need to be optimized to take advantage of multi-GPU environments. A poorly optimized game, or one that's too new to be optimized, will sometimes not benefit from the second GPU at all, and you're left with the performance of a single card until NVIDIA rushes out driver updates with those optimizations, which come out fast enough after big AAA game launches. An example of such is Wolfenstein: The New Order, which can't get itself to work with SLI. Some AAA games get optimizations even ahead of their launch. Not being able to properly play a game you pre-ordered for months, pre-loaded ahead of launch, and were keen on playing full throttle the instant it officially released would definitely suck.

The other more obvious requirement is the second PCI-Express slot, and power cabling, which shouldn't be a problem for mid-range ATX or even high-end micro-ATX motherboards. Bear in mind that not every motherboard with two or more long slots supports NVIDIA SLI, as motherboard manufacturers still have to license SLI support for specific models from NVIDIA.
 
Page 20, clearly shows that it the test setup used on the games tested;  SLI GTX 780 is better than single GTX 980

I would get 2 780's for the raw peformance.
 
1 980 if you feel that you dont want to be stuck with 2 780's buty later will buy a second 980. I have 2 gtx780 classified and have had them for a long time.