In general, SLI typically has an 80% increase in performance. So if you get 100fps now, then with SLI you should get 180fps. Of course, the real world isn't perfect so you might see more or less by some. A nice estimate would probably be 70-85%. Also note that if you have a game like Arma 3 or Minecraft mod galore and your getting low FPS, that is a CPU bottleneck and SLI will not help. The CPU in those cases are in a way, locking the GPU out because it's busy. Kind of like how when someone is really busy with something they might not hear what your saying when you talk to them.

But as mentioned above, Mwo doesn't support SLI. But other games do.