60hz monitor and gtx 980 gpu in sli plus an i5-4690k
you know part of me is weeping right now, and the other part is laughing hysterically? cause what i know about tw3, i know your 60hz monitor is a 1080p screen.
Dude. seriously? did you know right now someone with 1/2 the price of your rig is gaming on the same monitor in the same quality you are, to the point that you couldn't tell which was the $2000 gaming rig and which was the $1000 rig? You're really wasting all that money by using that monitor... heck there are people right now with a $90 amd cpu playing tw3 in the same settings as you are. Sorry i had to get that off my chest.
As to your question. vsync limits the frames coming out of the gpu to the max the monitor can display. your monitor can display 60fps (it's a 60hz monitor) so vsync is locking your cards to that limit, which of course is turning down the load on the cards. Those cards in SLi would hit 60fps in 1440p, or 144fps in 1080p... so your system bottleneck, as i was groaning about before is your monitor, as someone with a single gtx970 will hit the same 60fps in tw3 at 1080p.
I used to tell people that if they bought an intel they needed a 144hz 1080p monitor (at least) to get the most out of that intel, or they needed multiple 60hz monitors to game across a couple of monitors. else they might as well have bought a fx 8 core for half the price of their intel k series cpu. because there would be almost no end user observable difference between the two systems.
well you bought a gtx980, a gpu made for crushing 1440p and gaming on medium settings in 4k; you didn't just buy one, you bought two. with all that gpu power you really need a 1440p monitor, or even a 4k monitor. it's simply wasted in 1080p