Yes, you'll get a boost in fps, but not sure if the gains is worth the price for you. Go look up GTX 970SLi benchmarks vs GTX 980Ti's and then compare GTX 980Ti's to GTX 1080 and you'll have your answer

I've done the 970 SLI -> 980Ti -> 980Ti SLI -> 980Ti -> 1080 journey and I'd say that it's worth having a great single GPU over SLI. That's my personal opinion ofc, others may be more lucky than me with SLI. 970 SLI is a tad bit faster than 980Ti and 1080 is some 15-20% faster than 980Ti. 1080 is still not enough to give you 144fps at 1440p in all games but it's closer than 980Ti. I also have an Acer monitor (XB270HU) and I must say that the 1080 has left me a good impression.