It's amazing that Nvidia technologies interfere with one another.  I heard using the Geforce Experience and Shadowplay with a Gsync monitor is a bad idea.  Now you have Gsync conflicting with SLI in certain games unless you change the above mentioned Full screen only and Full Windowed mode around.
 
My experiences so far:
Witcher 3:  Pretty good scaling definite improvement in fps.  Minimal microstutter if you really look for it, but most people won't see it unless its pointed out to them.  This game has some weird issues on occasion, like yesterday, after taking a scroll through Novigrad with the fps staying in the 120s (FTW2, the 1080Ti's aren't here yet) I decided to go to another high stress area in the swamp.  FPS are in the 80s here and I see one of the cards clocking down to the 1300s while the other stays in the 1900s.  Funny too that the card in the 1900s had low utilization while the one running in the 1300s had high utilization.  Going to the map and back made it work normal again, go figure
 
Rise of the Tomb Raider:
For the most part the game runs great, however there are areas were the FPS drop down to the 50s and 60s with GPU utilization going down the drain as well.  This usually happens in the settlement area in the Geothermal Valley and also in the Mansion in the DLC.  It does it too with just one card, just not as severely.  Seems to happen more when the game is in DX12 mode
 
Tomb Raider Reboot:
This game is a post child for SLI and a good way to test the thermals of your system.  Pretty much 99% GPU the whole time.  Scales perfectly, looks amazing, too bad the follow up game isn't quite as well optimized as this one.  Made me play through the whole game again.
 
Mass Effect Andromeda:
Overall it scales very well, but I believe the game has some minor frametime issues with frames being displayed out of order on occasion which makes a game that runs in the 120s to 130s look not quite as smooth and fluid as it should.  Overall though, it runs very well and enables me to turn the eye candy up to the max, past the normal "Ultra" preset.  Microstutter is at a minimum, less noticeable than Witcher 3
 
Witcher 2
I know, it's an old game, but it had a mode in it's settings called "Uebersampling"  Turning this on using a 780GTX would bring the fps to the teens and 20s.  I revisited the game with one 1080 and now the fps hovered in the 50s.  In SLI fps are between 50 to 100...so yeah, great scaling.
 
Mad Max:
Ridiculous framerates.  No monitor can display what this game put out fps wise.  With Vsync off I had fps in the 260-300 range.
 
Dragon Age Inquisition:
Great scaling, no stutter, runs great.
 
GTA V:
Game runs great, but the sky does tend to flicker just a tiny bit in certain situations.  Overall, great experience and still a good looking game.
 
Far Cry Primal:
This is an odd one.  The game runs better when I disable DSR 2x which lands up being just under 4k when playing.  I got similar performance with one card as I did with 2 when playing at 1440p, but I got fps in the 100s when playing at almost 4k.  However, maybe a half hour into gaming the gpu utilization (which was at close to 50% when playing at 1440p) will suddenly drop from the 90s to the 60s and 70s along with drops in fps.  The game runs so well with one card though, it's probably the way I will keep playing it unless something changes with the Ti's in the system.
 
Dirt Rally:
Runs great with SLI, that's all I have to say
 
Watch Dogs 2
I am glad I got this game for free with one of the Samsung Evo 850s I had bought.  I don't like the game to begin with, but SLI performance is dismal.  Frame time issues like crazy and no improvement in fps.  I was willing to give this game a chance when they announced it has SLI support out of the box, but boy is it broken.
 
Star Wars Battlefront 2
Phenomenal, that's all I have to say.
 
Fallout 4:
Not a big fan of the game, but, for a Bethesda game, it runs great with SLI, no real complaints here.
 
Skyrim Special edition
Grrrrrr...out of the box you were able to disable Vsync which now is apparently impossible.  The physics of the game go nuts and you can't even make it through the opening riding in the cart sequence because your horse stretches for miles and the wagon flies, twists and turns enough to give anyone nausea.  With the patches came the lock to 60.  Well, with an engine that old you don't need to 1080s to reach 60.  Enough said.
 
Ryse:
Great looking game still and the difference between the PC and Xbone version is like night and day.  Scales fairly well, fps in the 120s, but GPU utilization in the 60s and 70s.  It's an older game and engine.
 
Dark Souls 3:
With it's lock to 60 fps, there's no surprise there is no SLI support
 
Assassins Creed Syndicate
Great GPU utilization.  Fps hovers around 100 most of the time, looking forward to see how this will run with 1 Ti's
 
I hope no one fell asleep trying to read this wall of text.