m8Xpayne: 144fps at 1080p you would see the difference between a GTX 1080 and a GTX 1070. In fact, asking for 144fps at 1080p is a bigger demand than 2k.
SLI is sketchy because it doesn't work with every game. You would see the difference for sure but that would depend on which game you're playing. The upcoming 1080ti and AMD Vega 10 would be better picks in your situation if you feel like you can wait. Otherwise, I would recommend getting a factory overclocked GTX 1080 and seeing how you like that.
would never recommend a single 1080 to anybody, never mind two. a single 1070 can handle 1080p and 1440p at 144 hz just fine. If you REALLY wanted 2 cards for some reason go with 1070's. 8 GB of ram is always gonna be a problem running at 4k, in that respect the 2 cards are pretty even
1080p 144hz? No. 1440p 144hz? Yes. If you haven't bought a 1080 yet you could also wait for the 1080 Ti, which will supposedly come out in 2-3 months and be as good as the Titan XP for around $800.
Depends what else is in your PC. Fine for high end gaming with a high end processor and at least 8GB ram.
However, if you have something like a Celeron of FX-4300, no point going that high end in video.
If you have too much money to spend, then yeah! Why not get one card, see if your games play just fine or struggle, then buy a second one later when it becomes cheaper ?