Ok, so here is the actual answer to your question OP but with some backup and another thing to think about.
A GTX 880 will NOT outperform two 780ti's in SLI in games where multi-gpu configurations are utilized. However, not all games are coded for multi-gpu setups. This is one thing many people do not realize. SLI/ crossfire does not provide a performance boost for all games. Many games do support this feature, however, as evidenced by some recent headline AAA games, this isn't always a ready at release feature. Example: Titanfall, T.E.S.O.
Your 780ti will likely be within 10% of the performance of the stock 880, so I wouldn't worry about upgrading this generation (I say 10% because it is a reasonable assumption to give the 880 a 15-205% advantage over the 780, and the 780ti is 5-10% faster than a 780) . If 780ti prices go down with the release of the 880, an sli option would be more viable in terms of a performance boost, but you should do research and ensure that multi-gpu setups are supported by your favorite games before taking this step.
Some people will tell you that multi-gpu systems are a headache and not worth it, however, if you read up on them, they are not that difficult. I currently run 2 GTX 780's in SLI with no problems and get a significant performance boost in MOST games I play, but not all.