Is 2-way SLi GTX 1080 worth it for UltraWide (3440x1440) build?

[nightwork]

Hi I am planning an ultrawide build with the 21:9 Acer Predator X34A 34" 3440x1440 100hz g-sync monitor.

I am finding it hard to figure out how much FPS improvement SLi 1080's would be over just one 1080.

All the benchmarks I can find comparing one 1080 to SLi 1080's seem to focus on 2560x1440 and 4K.
CPU will be: i7-6850K

Also, a video I watched said the 1080 SLi performance should improve over time with drivers and game development.

Could anyone give me advice here?

I'd be happy to sell 1080('s) and upgrade to 1080ti in ~Q4 when they come out, but don't want to drop £650 more now for second 1080 if it's not really going to be worth it.

 

[nightwork]

Wow thank you all for your enlightening replies, interesting stuff.

I think i'll take the consensus of advice and stick with one GTX 1080 to start with; as it seems to be really dependant on the games etc. No harm in adding one later as you say (or maybe a Ti by that time).

Much appreciated.