As he said, you can get maybe 5-20 more fps on some titles at 4K. But in some other cases, you get less. And you may end up saving money selling your old GPU and getting a new 1080ti unless you can get a really cheap used identical card as yours.
You would also need more power, 2x6 pin 2x8 pin connectors and a quality PSU with at least 750W. Also from what I've seen previously to 1080ti release, the 1080 and 1070 aren't that good at dx12 while the 1080ti is as good as AMD cards on dx12 (yet more powerful than AMD cards).
For the small boost of fps over the 1080ti and LOSS on some games that aren't SLI optimized, it's not really worth the pain in my opinion. The 1080ti can give you almost 40% the performance boost without any pain.
Unless I'd find the same GPU as yours, on a huge sale, I'd probably not do it. Either exchange it for a 1080ti non-reference card, or wait for 2080 (then prices drop and you can SLI cheap or upgrade if it's a lot better) but in my experience, the 980ti was almost 10% slower than the 1080 so it wasn't worth the upgrade since I could OC.
Try to sell the gtx 1080 and get a gtx 1080ti .Sli is waste of money and lot more power unless it scales well ,which is a rare case
What do you mean by "unless it scales well"
Not many games / applications take advantage of SLI well. Some only perform slightly better with SLI as opposed to one GPU, some perform the same, some actually perform worse. Very few games will properly utilize both graphics cards in SLI.
Why is everyone talking monitors? Am I missing something or is this a GPU discussion? The only thing I'd change is get bigger monitors for 4K since 28 is the limit at which you can actually see a huge difference in resolutions. But not really worth buying 3 new monitors. And gsync isn't really that great, I'm disappointed by it.
I have 2 x R9 Furys in Crossfire. They actually perform/scale well in most of the games I currently play, but newer games from 2016/17 often don't support multi-GPU at all - which is why I no longer recommend it to the average user.
I'd carefully check to see if SLI (or DX12 m-GPU) is properly supported in the games you play before getting a second card.