SLI performance, right now at least is all about vendor support. You need Nvidia to provide an optimized driver to take advanatage of it. That usually happens within a week of a popular game being released, but it's possible that a particular game will never get a profile, or won't get one for months.
Framepacing issues leading to microstutter I think will improve drastically with upcoming hardware and Gsync. So I too wouldn't recommend it over a more powerful single GPU right now, but buying another card to pair up in the future (especially if you've got a Gync monitor) sounds a lot more attractive once the 800 cards hit and Gsync monitor become more readily available (and cheaper).