A 1070 will move 1440p just fine at 1440p. The 1080 is a 20% performance improvement for a 54% increase in price. If that's what ya thinking, wait for the 1080 Ti to drop which will push the 1080 / 1070 south. With the 980 Ti's release, the 980 became a forgotten card. I expect the same with the 10xx series.
As for SLI, that's a bit of a conundrum. When I see highly negative comments about SLI, and ask in what instances did you experience this, the most common answer is "I read it on the internet. On the 970, simply put, it was the proverbial "no brainer". This was one of the reasons that the 970 was the biggest selling card of all time, out selling all AMD 2xx and 3xx cards combined. When you can buy two 970s for the price of a 980, and the twin 970s are 40% faster on average (70% average scaling including games that didn't support SLI, demanding games in excess of 95%) than the 980, the 980 lost its luster.
On 10xx as well as AMDs 4xx, it's quite a disappointment. many reasons have been suggested for this:
1. GPU advancements have been generation to generation have been as high as 50% or more, whereas CPU advancements have been paltry. To do SLI and keep minimum frame rates up, the system performance is affected by RAM speed, CAS and CPU performance.
2. The driver teams for both sides, are struggling to keep up with games that started as DX11 and now are rushing to get out adaptations for DX12. As always, work on SLI takes a back seat to single card development.
3. Does nVidia really have a interest in improving SLI performance ? As it stands now, the 1070 and 1080 have no competition from AMD. So given the fact that card manufacturers make far more margins on their flagship than selling 2 lesser cards, does it really make sense to improve 1070 SLI performance ? The only card sales that would be hurt by better 1070 performance in SLI would be that of the 1080 ... so until there is something to compete with these cards, they would be shooting themselves in the foot so to speak by improving SLI performance.
Looking at Witcher 3 for example at 1440p, we see about 64 fps... there's not many more demanding games around and if 60 fps is your goal, the single 1070 gives you that. You will take a loss on the 1070, maybe get $300 for it, and then ya gotta but a $650 card. Is 13 more fps worth $350 ?
a) Stick with the 1070 *
b) Right now scaling @ 1440p is about 33% over TPUs entire test suite... keep in mind that games w/o SLI support of without SLI support yet are included. Stay put right now and watch to see if things improve and the driver developers catch up and the cards have some competition to incentivize the time investment for improvements.
c) Look again when the 1080 Ti drops