My son has the Acer Predator XB270HU (IPS 144 Hz, 3 ms lag time) and it just rocks at 1440p on twin 970s. The 1080 is faster than twin overclocked 970s get 75 fps in Witcher 3 which is what i generally use as a source of comparison. The 1080 gets about 73 fps but, while the current FE cards **all** throttle, the 3rd party non-reference cards (well most of them) will not.
http://videocardz.com/60838/msi-geforce-gtx-1080-gaming-x-is-much-better-than-founders-edition
NVIDIA’s own reference design suffers from severe throttling just after few minutes. It probably wouldn’t be that bad if not the frequency spikes. While average clock is somewhere around officially stated boost clock, those spikes cause micro-stuttering, which negatively affects gaming experience.
Hardware.info:
Founders Edition suffers from a horrendous amount of throttling and it runs +- 150 MHz lower all the time.
Meanwhile, MSI GTX 1080 GAMING X generates almost a straight line for GPU frequency (~1910 MHz), with no spikes and rather constant sub-70 C temperature. This should mean that the gaming experience will be much better, and card should theoretically generate better results in most tests. Also according to H.I. this is also the best custom design they so far tested.
The non reference 980's OC'd about 25% ... **if** we see the same increase, one 1080 should deliver 90+ fps in Witcher 3 which should allow you to use ULMB on almost any game. There's a review of the XB270HU here:
http://www.tftcentral.co.uk/reviews/acer_xb270hu.htm
The newer model is the XB271HU ($717) which differs from its predecessor in that it is 165 Hz and it allows for a 120 Hz frequency under ULMB.
https://pcpartpicker.com/product/ttnG3C/acer-monitor-xb271hubmiprz
My sons shares a house with one of his college buds and he has a 34" panel powered by a 3440 x 1440 screen powered by twin 980 Tis. The wider screen has the twin 980s delivering about the same fps as my sons twin 970s at 1440p. Comparing them, I much preferred the 1440p Predator mainly due to the Predator being a low lag IPS screen. NOTE: That is not a wholesale recommendation for IPS screens ... the quality of panel required for gaming, is expensive. I have yet to see an IPS screen under $600 that I would use for gaming.
I was anxious to see how a 3440 x 1440 IPS screen would do and the X34 is the one with which I am most familiar...the $1234 price is a bit off putting ... as is the 25% fps drop from 1440p.
But the X34 panel, like all 3440 x 1440 panels, doesn't do ULMB. Until we see monitors on the market supporting Display Port 1.4 (1080 has DP 1.4), that's not going to happen.
If you are not familiar with ULMB, the price premium for G-Sync capable monitors comes primarily from the ULMB hardware module not G-Sync. Freesync has no comparable technology. ULMB or Ultra Low Motion Blur technology just about eliminates ghosting and if you are getting above 75 fps, which you will in almost every game at 1440p w/ the GTX 1080, I think you will find using ULMB will deliver the better experience. See demo video below
http://www.blurbusters.com/zero-motion-blur/video/
Here's the review on the X34.
http://www.tftcentral.co.uk/reviews/acer_predator_x34.htm
In short, while the curved 3440 screens do provide a sense of immersion, I find them annoying when used for other work. But even in gaming,
giving up ULMB is just too big a sacrifice for a wide 3440 screen ...and although the situation is improving, some games still have issues with that resolution.
For that reason, to my eyes the choice comes down to the Acer XB271HU and the Asus ROG Swift PG279Q offer the premier gaming experience available today.