mrjenkins44 :
So the monitor would just be for future games? Would I even be able to see a difference in the games right now?
The bottleneck in your current system is the 60fps cap (or 75fps, if set at 75Hz) of your BenQ RL2455 monitor. Your GPU is very powerful that it was intended for 1440p (or even 4K) gaming. Using a 1080p with a cap/limit of 60Hz/75Hz means that your GPU is not being used to its full potential. On your current monitor, without V-Sync On, your GPU is rendering more than 60 or 75 frames per second that your monitor cannot display due to its refresh rate, what happens is frame stuttering or screen tearing - which might be an indication of your "slowdown?" as you mentioned. Buying a "bigger" video card on your current monitor would worsen that effect.
Consequently, if you turn V-Sync On, your GPU will only output 60/75 fps and disregard displaying the other frames which would make gameplay much "better" - but, as said, the GPU's not reaching its max. potential due to the 60/75 cap limit.
Note that your current CPU can handle such powerful GPUs (even a GTX 1080 or 1080Ti), so, no need to worry about overhauling your rig.
I would never personally consider (nor recommend) going for an SLI setup. It's not worth it as only a few games support such. Most games, you won't see any performance difference. Some games it may have a slight performance increase. Some games even works better with single GPU than multi-GPU (worse performance in SLI).
So, personally, if you are considering upgrading your GPU, I'd keep your current CPU/MB/RAM/SSD and invest on a better monitor first (to maximize your current GTX 980 Ti). Then later on, as newer and more powerful GPUs come along (with better prices than it is now due to the mining craze), you'd be in a better position to select a more appropriate GPU at better prices with a quality monitor at hand.