We were at $1600 i recall did the extrta money go to SLI. SLI i find it useful for 4k gaming on 2 screens and still good FPS. SLI also helps because each GPU renders a part of the screen so the GPU isn't maxed out which is bad. Think of 2 cores. 2.5 GHz dual core is better than a 2.5 Ghz single core at Multitasking And Multicore Usage maybe equal in single core performance.
Many people say the human eye can't notice differences in the upper 60 + FPS or it cant process maybe that much Fram Rates Per Second.
Well think of your eye and you move around do you see any lag? Nope Smooth on out. For example blink. As soon as your eye opens after blinking you see an image, thats because your brain processes it very very very fast , you can't tell how fast it does for example 1 ms