I had 2x480 in SLI mode, and heat was serious problem. I could not get thing cooled, i was around 110C (230F) after 10min of 3d play. So no more SLI fun for me it is not worth the trouble. Also what wrote: ue4 has no sli and many other games. Then you have that funny stuff like 2 frames being rendered together then usual wait for next 2. So your fps counter is high but in reality you have half of it. Not sure if was fixed in drivers, i did not bother with sli since.
And GF 980 here is 860$, about same as i got nvidia shield for, evil greedy gov with their robbery.