Pros and Cons of SLI

Pros:

Bonus performance
Looks cool

Cons:

Not supported in some games but since you're getting 1080 Tis that's not really something you can deal with

There's really nothing else with Pascal. Heat and power aren't really issues, and the weight is on individual PCIe slots more than the whole mobo. Some cases (e.g. Coolermaster MasterCase 5) also offer a GPU support brace that you purchase separately.

At the moment you could get PCIe slot support braces (there's one called the "Atlas" and an acrylic one from Puget Systems) that screw into the bracket at the rear of the case, but the compatibility of those with 2.5 slot cards is questionable. Other than that it really just gets specific, such as the braces built into the Anidees Crystal or the add-on brace that slots into the MasterCase.

. Very nice rig btw man

Just as an FYI--there's no way that even two 1080Tis or Titan XPs for that matter will be able to take advantage of the 144Hz refresh rate on that monitor. A single Titan XP is only barely breaking the 60FPS mark in most games on ultra in 4k. Add in that you don't always get perfect GPU scaling, and that means probably under 100-110 FPS in 4k. I would say (if you're dead set on a 4k monitor) to grab a 60Hz G-Sync monitor or bump it down to 2k or a 2k ultrawide with a 144Hz refresh rate.

As for the rest of it--yes not all games support SLI, so there's a bit of extra tinkering to be had, but you can choose whether or not to run a game in SLI. You need not take a card out of your machine if you don't want to run SLI, there are options to disable on a by-game basis. As for the rest of those pros/cons--that about sums it up. You get far greater performance at the cost of thermals and space. Looking at your motherboard, as long as the 1080ti isn't a triple slot card, you'll have space for the M.2 underneath the first x16 slot.

So, at that point it's really up to you whether or not it's worth it. There will be games that don't benefit from SLI, so at that point you have to ask yourself what games am I playing?; if the majority of them are AAA titles, they will support it, and they will support it well. If the majority are indie steam 8bit games, they probably won't support it/need to support it. If you feel like you'll use the increase in performance from SLI, it will end up being worth it. If the 1080-1080ti-Titan XP hierarchy is anything like the 980-980ti-Titan X hierarchy, the 1080ti will come close to the performance of the Titan XP.

A single 1080ti will suffice for 1440p gaming, and you shouldn't be messing with 4k as it's too much resolution for 27" and you would have to use high DPI settings in Windows which sucks to put it mildly.

I've done a few SLI setups for my self over the years: 560ti, 970, 980m, and now 1080.

You'll find many give grief over them but I currently only have one out of 70 games that don't support it(Batman AK). Overall you will, however, need to wait about a 1-2 months or so after a AAA game release to gain support and usually another month for them to better optimize. 850w is cutting it close, 1000w would be closer to the sweet spot.

Some reviews to check on 1080 SLI:
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_1080_SLI/3.html used 1200w
http://www.kitguru.net/components/graphic-cards/zardon/msi-gtx-1080-gaming-x-8g-rgb-sli-review/2/ used 850w
http://www.guru3d.com/articles_pages/geforce_gtx_1080_2_way_sli_review,7.html used 1200w
http://www.overclock3d.net/reviews/gpu_displays/overkill3d_-_rog_swift_pg279_triple_screen_test_with_strix_1080_sli/1 used 1000w

I think you'll really have a lot of fun reading that last link. There's some reviews on Titan X SLI with mostly the same PSU's. You'll find many people jumping on the bandwagon stating that SLI is dying. There's some truth to that but its also true there's no such thing as a single GPU(for under $2000) that can solidly go over 4k/60hz all the time. For that reason alone SLI will have some life for years to come.

Info for settings I use on most games in 4k: No blur of any kind, whats the point of obsessing on 4k just to add blur. My monitor is fixed 60hz refresh so I do usually use V-Sync since again the point of 4k is clarity so why allow tearing. I tinker with AA. Early 4k display reviewers stated you shouldn't need it but sometimes it still helps. There's so many different kinds and they can still slow you down a bit, but I do occasionally see improvements.

My 1080SLI will hold ~60-80 FPS depending on the game. ROTTR can still bring 'em down to 30-40 if I max everything but with no AA or Occlusion and everything maxed it goes back to 60. GTA V in similar settings performs about the same. Witcher 3/DOOM/MORDOR at full max go 60 or over. Metro's maxed except .5 AA will hold over 60. I just remembered and tested Assassins Creed IV yesterday. Game looks awesome at 4k and held ~60 with all maxed. So bottom line here is that if you really are going to 4k/144hz down the road 1080SLI probably can't go a whole lot higher that 60-80 w/o significant compromises.

For your display choice I really have to say go bigger. 27" is going to be hard to see a difference on. 30" is around the minimum that 4k really begins to show its stuff. I don't know if Asus is going to go bigger for that 4k/144hz but they should. Eventually I'll probably upgrade to that in a year or so.

I have the pg278q and love it for 1440p. 1080SLI will hold 120-160 with the same settings I mentioned above. G-Sync does rock! Especially on 144hz 1ms response displays. 1080SLI pretty much owns 1440p. No problem.

It kind of hurts me to recommend going for a Ti SLI since you already have a 1080 but . . . . . .if you have the money and really want to do and are committed to 4k/144hz then go for it. That will be the strongest GPU setup anyone is going to have for a good while and I'm sure you'll be able to find a good home for that 1080. There's already some complaining about the rumored slower vram for the TI's but some have also stated that even those speeds are more than enough. It almost always comes back to clock speeds and cores. VRam size is more of an issue. I've been monitoring mine closely and with all settings maxed at 4k some games hold around 6-8gb while others are 4-6. Games from before 2014 usually top 2-4.

For system ram you might need to go to 32gb. Most games 2014-2016 are using 8gb but there's a growing list of ones from 2016 using 10-12gb at 4k. It's not totally common but it is happening.