Once DX 12 games hit it may be worth SLI'ing your card, IF you don't mind the nonexistant privacy with windows 10.
Windows "effectively" adds GPU memory together under DX 12 ( in theory). We'll see.
http://www.tomshardware.com/news/microsoft-directx12-amd-nvidia,28606.html
Part of this new feature set that aids multi-GPU configurations is that the frame buffers (GPU memory) won't necessarily need to be mirrored anymore. In older APIs, in order to benefit from multiple GPUs, you'd have the two work together, each one rendering an alternate frame (AFR). This required both to have all of the texture and geometry data in their frame buffers, meaning that despite having two cards with 4 GB of memory, you'd still only have a 4 GB frame buffer.
DirectX 12 will remove the 4 + 4 = 4 idea and will work with a new frame rendering method called SFR, which stands for Split Frame Rendering. Developers will be able to manually, or automatically, divide the texture and geometry data between the GPUs, and all of the GPUs can then work together to work on each frame. Each GPU will then work on a specific portion of the screen, with the number of portions being equivalent to the number of GPUs installed.
As for privacy tradeoffs:
http://www.tomshardware.com/news/cortana-is-watching,29791.html