I'm afraid that is not my experiences. I use 2 x 980Ti (MSI) in SLI and both cards are working hard to give me 75fps, 1.5ss and most of the setting to high or ultra. When I only had a single card, I had to have almost everything OFF or LOW in order to get the 1.5/2.0 ss enabled.
Downsides of SLI though I've noticed in the game... DSR cannot be used as it causes massive flickering in ED for me. I have to use in game SS, the debug/mirror screen (AppConfig.xml) doesn't work if I have SLI enabled. ED fails to even load up (any one else get this?).
Whether or not you can justify a £600 extra GFX card for what probably amounts to only a 30% gain at the moment is another thing however when Nvidia and especially FD specifically support and optimise for both a HMD and SLI then we should all see a massive boost.
If you don't play with a HMD and only use monitors then 60fps is fine and I doubt SLI would help you as a single 980Ti can handle that no problem and likely even at 4K.
If you are getting low fps with everything set to low at 1.5ss on 1 card then something is wrong with your setup.
My card only dips to around 75fps when im in a station and 100fps anywhere else (capped), i can also post videos to prove this with realtime OSD running.
I suggest you use Nvidia Inspector because it sounds like you aint using it: so you get no custom SLI flag therefore no proper utilization of the second gpu.
Something is not right with your setup, maybe you should also try installing gpu driver again but dont forget to use DDU to remove old remnants, very important this is and stop relying on nvidia control panel for your gpu settings because most the time even when you set them up it dont work and has not worked for years (5+)
Maybe your cpu is bottlenecking your cards, if your Cpu utiliztion is maxxed out at 95-100% then your cards are not being used to there full potential.
If i had another Titan X i would be hitting around 200fps or more in some areas of the game easily with my rig.
Same as Batman Arkham Knight and Fallout 4, i didnt suffer from the problems with Batman that others suffered and i can also post a video. Fallout 4 im not even using an Nvidia optimized driver for it and i get 100fps and drops to mid 70s. I have windows 8.1 massively tweaked and i DO NOT use any driver that has shoddy windows 10 data included within it. Im on 353.38, works flawlessly with anything.
Im also referring to 1.4.1 and NOT Horizons.
Edit....
I just tested Horizons and landed on a moon, its taking up 5160MB of Vram and is reducing the core clock on my card. Its very demanding, another step up.