AHowes2 1080s will be awesome for either 1440 or 4k. A single Titan I hear is not enough for 60fps in 4k with everything maxed.
The Titan is only 20-30% faster then a 1080. If that's worth an extra $500+ then go for it.. but 2 1080s in sli will drag that Titan in any game that uses sli.
In anything but a AAA game released in the last year or so I can peg 60 fps at the highest settings with my single OCed 1080 SC in my signature rig with my 4K monitor. And some AAA games, like DOOM, thanks to the Vulkan library I'm up more like in the 90s at absolute ultra (my monitor is limited to 60Hz though).
In The Division I'm around 60 at highish settings, and with absolutely maxed ultra settings it's like high 40s. The quality sacrifices to get from the 40s up to 60 aren't actually that bad, mostly things like ambient occlusion, keeping AA off, and possibly some shadow quality. With my Xeon system to get up to 60 fps my shadows were so low quality that they looked blocky and pathetic, since my rebuild with the 6900 I'm up close to 60 with shadows at a little higher quality so that they don't have to look blocky anymore.
My goal is to pick up another 1080 SC in the next three months or so (maybe for Christmas) and go SLI, and to that end I picked up the EVGA HB bridge recently when it became available. This will absolutely peg any game I currently own. I'm hoping that sometime in the next couple of years I'll be able to get a 120Hz 4K g-sync monitor, and that the 1080 SLI setup will at least get me up around 90-100 fps and look nice on a monitor like that. Plus a second 1080 will make a good folding rig into quite the folding monster.
Anyhow, I'd love to have a Titan XP instead of a 1080, because that would actually peg anything I currently play at ultra with 60 fps at least. Even if it's only 20-30% faster, that's still enough to close the gap to 60 fps in pretty much any game. And Titan XP SLI would go a long way toward pegging a 120hz 4K monitor as they become available.
I'm just worried that I'll fork out for a 2nd 1080 and then a few months later a card will come out for a similar cost that would do most of what a 1080 SLI setup can do in a single card. Single card solutions are always better, not just because SLI isn't universally supported, but also because SLI setups have greater frametime variance, for a reduced quality (whether it's still noticeable or not I don't know, but it's measurable).
Anyhow, my hat's off to anyone who forks out for a Titan XP. I mean, if they have the money, why not? And if they don't, it at least gives the rest of us something to daydream about. I bet with VR headsets starting to come into use, a Titan XP would rock that. Vive + Titan XP sounds like a winner.