Its a VERY demanding game to be fair. Not saying this is your problem but a lot of people think games are unoptimised when they are just trying to run it higher than their system can handle. Sli could be your problem as a single 980ti wont max it by itself and SLI support is...well, SLI support. Ive had to turn some stuff down on a single 1080 @3440x1440 as its really demanding. you can tell just by looking at it though as its a truly gorgeous game
I wouldn't say it's all that demanding, many people drop to settings their rigs should easily run but they still have problems. There are clear problems with coding. I'm on a 1070/4790k and I can't always sustain 60fps even after dropping some settings and there's some annoying hitching, especially in the city.There's practically no difference fps-wise between msaax2 and x8 or 4k DSR, which points to some problems with the engine.
Also, I don't think the graphics are anything groundbreaking. All I see here is great looking cars and skies, the rest is pretty plain. The water, shadows and some vegetation are pretty bad. The map isn't that big (a bit of an understatement, after driving around it for a while I think it's pretty small) and doesn't feature any complex physics or a lot of objects. Most of the looks come from lighting/filtering.
GTA V pulls of a very good looking city, better water and vegetation, better physics and a lot more objects on screen with a higher framerate and is based on a pretty old engine.
I really think this game should be no problem for fairly modern hardware.