It would be worth it, yes there are some games which are not optimized and not as much scaled, but most do. And they do give you a worthwhile performance boost. If you really want consistent performance go for a powerful single card. But if you go for a SLI you will also get performance which is worthwhile but not across all games. Ultimately, the decision is yours to make.
If you have already have a single card, which you do, it is cheaper to just grab another card for cheap and get a substantial performance boost. It will give you a boost in most games, but will not guarantee a boost in all games.
If you wanna go with one single powerful card, then its likely you will need to spend more in order to get that card. But, in my eyes its not that worthwhile to get just 10-15 FPS increase with buying a new $300+ card instead of just getting that same performance increase with just $160+.
You do also have adequate power for that SLI.
As I said it's your decision to make. If I were you I would go for SLI and when I need to upgrade once again, presumable next year I would buy the new maxwell cards. Possible the 870. So.. yeah