Well, I got an autonotify that the 1080 ACX FTW was in stock on the EVGA website.  I went ahead and ordered in Friday.  Right now, I am still sporting a pair of reference EVGA 780 TI's in SLI.
 
I only really play BF4 in 4k these days.  Right now, the 780TI's are actually doing the job pretty well considering that they are 3 odd years old.  I get 40-80fps depending on map/mode and player count.  I have AA off and resolution at just 110 at 3840x2160.
 
I had not seen a direct comparison between the 780TI and the 1080 due to the generation gap, much less an SLI comparison.  Many are quick to estimate in terms of percent of benefit given relative benchmarks from 780TI to 980TI to 1080.  Of course, actual feel and function will be the real determination of whether or not it is "worth it."  I will report back in another week or so as far as how this new card compares to what I have right now at stock clocking levels.  Down the road, I will probably get a pair of 1080TI's in SLI when available provided it is a meaningful upgrade.
 
Other relevant stats are :
i7 2600k @ 4.2ghz (this guy is still a great cpu. amazingly enough considering it is 6 odd years old??)
16gb ram
win 7 64

like I noted in the OP, I am at 4k.   I wasnt terribly clear there though.

Nice, I appreciate your input.  Something I do get that is kind of irritating is occasional BSODs due to GPU.  Hopefully this will get rid of that.
 
More to come

redteamgo
I received the card yesterday
 
Anecdotally, the single 1080 FTW is definitely faster and definitely more quiet.
 
The basic initial comparison was stock for stock where I am about 100fps @ 4k with AA off and resolution post processing at 100.
 
When I use the 780 TI in SLI, I typically clock around 50.
 
When I dial up AA to 4x and resolution to 130, I land around 60fps +/-5
 
If I apply +75 core and +250 mem, I land around 70-80fps on average.
 
The different map modes and whatnot definitely make a difference.
 
All in all, definitely an upgrade but not a great leap forward versus the 780 TI's.

Yes but you are comparing two cards in SLI versus a GTX 1080 FTW. The real comparison would be 2 GTX 1080 FTW in SLI versus the 780ti in SLI.

Don't forget about the power draw (and heat) and the noise delta.

Ok, so I just had the 780ti in SLI and have moved up to the 1080 FTW in SLI. I have not had much of a chance to game with them, but I can share a couple 3Dmark scores.
 
With Fire Strike Ultra on the 780ti's I got 3871 (medium OC)
http://www.3dmark.com/fs/8580799
 
Then with just the one 1080 FTW I got 5553 (slight OC)
http://www.3dmark.com/fs/9436850
 
And then with both 1080 FTWs in SLI I got 9599 (slight OC)
http://www.3dmark.com/fs/9463036
 
Oh but man one of the biggest differences is the noise, or should I say lack of it. These cards are so quiet. Hope that helps with the comparison. Later today I might be able to provide some frame-rate comparisons for Witcher 3 as well.

ipaine
Ok, so I just had the 780ti in SLI and have moved up to the 1080 FTW in SLI. I have not had much of a chance to game with them, but I can share a couple 3Dmark scores.
 
With Fire Strike Ultra on the 780ti's I got 3871 (medium OC)
http://www.3dmark.com/fs/8580799
 
Then with just the one 1080 FTW I got 5553 (slight OC)
http://www.3dmark.com/fs/9436850
 
And then with both 1080 FTWs in SLI I got 9599 (slight OC)
http://www.3dmark.com/fs/9463036
 
Oh but man one of the biggest differences is the noise, or should I say lack of it. These cards are so quiet. Hope that helps with the comparison. Later today I might be able to provide some frame-rate comparisons for Witcher 3 as well.

Some really good info so thanks for sharing. This shows a really good difference between the two in SLI.

ipaine
Ok, so I just had the 780ti in SLI and have moved up to the 1080 FTW in SLI. I have not had much of a chance to game with them, but I can share a couple 3Dmark scores.
 
With Fire Strike Ultra on the 780ti's I got 3871 (medium OC)
http://www.3dmark.com/fs/8580799
 
Then with just the one 1080 FTW I got 5553 (slight OC)
http://www.3dmark.com/fs/9436850
 
And then with both 1080 FTWs in SLI I got 9599 (slight OC)
http://www.3dmark.com/fs/9463036
 
Oh but man one of the biggest differences is the noise, or should I say lack of it. These cards are so quiet. Hope that helps with the comparison. Later today I might be able to provide some frame-rate comparisons for Witcher 3 as well.

nice post thanks.  this is the kind of comp i was looking for before I bought.  I am still happy w my purchase and will SLI the TI when it comes out.. doesn't seem worth it yet.
 
I definitely agree on the noise.  my system is far more quiet

never mind.

Ok, so as I mentioned here are some results from Witcher 3.
 
For the settings, the 780ti runs were made with everything maxed out except two things.

1. Hairworks. It was set to just Geralt and hairworks AA 4x. 
2. Foliage Visability Range was at high

For the 1080 and 1080 SLI runs everything was completely maxed including hairworks for all and at 8x AA.
 
All tests run at 2560x1440
 
780ti (5 runs, lowest Min, Highest Max, Total average)
Min  Max  Avg
37   64    48.3
 
1080 (8 runs, lowest Min, Highest Max, Total average)
Min  Max  Avg
42   89    62.3
 
1080 SLI (8 runs, lowest Min, Highest Max, Total average)
Min  Max   Avg
55   110   84.3
 
The actual runs themselves are taken deep into the game. There are stretches of just riding through the wilderness on horseback and through towns, as well as combat with bandits and furry creatures and others. I'm pretty much positive that on the 1080 SLI run, the lowest min of 55 was achieved when I was fighting an entire pack of wolves in forest with lots of brush. Considering hairworks, I don't think you could get it to be more demanding.
 
And lastly here is a graph showing the fps throughout the 90 second runs.
 

 
So a lot more dips with the 1080 SLI compared to the 780ti SLI, and I'm not sure why. Maybe it has to do with hairworks being cranked up, but not really sure. Anyways there you all go.

Glad I found this thread! I too own two 780 ti and just purchase a single 1080 FTW : http://www.newegg.com/Product/Product.aspx?Item=N82E16814487245
 
I game a lot at 1080p due to the capture card I use for twitch streaming. But when I'm not streaming, I game at 1440p w/ G-Sync and it appears this card can handle it's own at that resolution on high settings. Maybe down the road I can grab a second card for SLI setup.