So far, you don't seem to have overly complex scenes going on, but if you are looking to the future for your Daz Studio usage, the 1080Ti is definitely something to consider, due to the 11 Gb of memory. You won't want the Titan XP though. It's almost twice the money of a 1080Ti for not much gain. Sure it has 12 Gb of memory, but two 1080 Ti's will give you much more bang for your buck.
$730+ for a founders edition 1080Ti might be out of your budget though. It IS significantly faster than a stock 1080, sometimes as much as 30% or more faster...
1080 Ti's are the holy grail for Daz Studio these days. Titan XPs are about 5% faster, sure, but you are paying almost double for them..
Also, as pointed out, keep in mind that VRAM does NOT 'stack' with multiple cards, as each card must hold the entire scene within the card memory in order to work on the scene. But the doubling, etc. of 'cuda cores DOES make a difference (roughly half the render time with 2 cards, 1/3rd the time with 3, and of course 1/4 the time with 4). It's not 100% linear, but it's pretty close in the benchmarks.
As to whether you are going to be able to use all 11 GB of VRAM (actually 9, Windows reserves 2), well you'd probably need a fairly complex scene with multiple characters, and maybe HD textures, to hit that bar, but it is possible... And of course, you can always add a 2nd card later to reduce render times (and maybe 3rd or 4th, after that the diminishing returns make it not worth it).
Again, this is a 'building with the future in mind' choice. If your budget will allow. Should you decide that you like this rendering thing, you might as well get the best you can afford, when you can afford it.
You won't necessarily need SLI for Daz though. Two identical Nvidia cards should work, but the 960 that you are using now is pretty far down the food chain as far as Iray rendering goes. Might as well get a faster/better card instead, as you are planning.
You MIGHT be able to do something weird like use the 960 to run the system display and Daz viewport, while reserving the faster card exclusively for rendering... I've used my integrated GPU to drive the Daz viewport before (in my previous system, it was AMD graphics), while the CPU focused on the rendering (didn't have an NVidia card).
Your game is off to a very good start, so I suspect that patrons will continue to flock to your efforts. You'll be able to afford better cards soon enough.
