If you look at the review I posted it has the 870M running battlefield at ultra, 1080p getting average 37.1 fps. Around 30 fps is what most people consider the minimum playable frame rate, and above that everything just starts to look smoother. Below about 25 fps is where you start to see stuttering. Personally if i had a 870M I would just play on high settings, but if graphics are more important to you it is would still be playable on ultra at 37fps.
Yes the Battlefield 4 does support SLI and Hardline should too. Sometimes games have compatibility issues with multi-card systems at launch but it's usually patched, although on occasion it's never fixed. It's usually better to get one more powerful GPU than two weaker ones, even if the two combined will be slightly more powerful, although in your case this isn't really possible.
The X7 v2 uses the Kelpar 860M's by the way.