I'll try my best to answer as many questions as possible here:
If you bought two 980ti's, your FPS would improve dramatically, yes. Although they are expensive cards, and one is enough to run everything right now, in 4k, on max settings, at 60+FPS, so it's not really necessary to have them in SLI for a while.
Your current rig would maybe need a couple of extra case fans if you wanted to add another 980ti at a later date. Generally, nVidia cards run at very respectable temps, however the sheer size of having another card will mean there is much less space for air to get around your case. Nothing too serious though.
(And yes, your case would be big enough for an extra GPU
)
Finally, on the subject of your two 144hz monitors:
It's not a question of 'whether or not a GTX 970 can run two 144hz monitors'... It's whether or not a 970 can run games in high enough frames to take advantage of the 144hz monitors.
These two quotes explain it better than I ever could:
"There is a huge difference between 60hz and 120 or 144hz when it comes to smoothness and feel. Most die hard gamers understand this and perfer this. The problem that most people have is that they buy a 120hz/144hz monitor and they run a crappy video card that can't run high enough fps in the games they play to match the hz rate so they don't feel the benefit of the 120/144hz monitor. If your a 1080p gamer playing the latest games, you better have a really good video card or sli setup that will allow you to average at a minimum 110-150 fps in all your games otherwise don't buy the monitor."
"if your gpu is capable of delivering 120+ fps, then the gameplay will be smoother. You will notice the difference, no doubt. If your gpu cant do 120 fps, then dont bother, because you wont be able to enjoy the full experience of such a monitor.
I should say this... before i had 144 hz monitor, i didnt think there will be much difference, but after having it i can't go back.
the 3d difference between 60 hz and 120-144 hz is muuch more observable indeed.
PS : professional games often suggest having as many fps as posible, even by lowering quality. they would not recomend 120 + fps if they had a 60hz monitor."
These two quotes were taken from here:
http://www.tomshardware.co.uk/forum/id-1845694/144hz-monitor-worth.html
To answer your question, a single GTX 970 will easily be able to run LoL in 1080p at over 110FPS, so it will take advantage of your 144hz monitors, however it won't run high-demanding games at such a high FPS, so in those cases, the 144hz monitors won't have any real benefit over a cheaper 60hz monitor. But if you already own those two monitors, then forget all this. You will be fine. On low-demanding games, they'll be fantastic on a 970. On high demanding games, a 970 will run at around 60-70FPS and so the 144hz monitors will still look great, but just look like normal 60hz monitors. (which personally, I find fine, I have 3 monitors at 60hz)
I hope this helped