In the passmark benchmarks, which runs Direct X baserd benchmarks, the TitanV (Volta) only punches 5.3% better than the 1080ti, with 14,440 vs 13,701. So mild overclock of the 1080ti should even things up?
https://cdn.discordapp.com/attachments/339810774501294087/397339381515681802/unknown.png
Taken From:
https://www.videocardbenchmark.net/high_end_gpus.html
https://cdn.discordapp.com/attachments/339810774501294087/397338807705403393/unknown.png
And the above screenshot is from a benchmark I ran a couple of days ago on my machine with a 1080ti to see how the fresh copy of windows was running. I run my GFX card on the quietest " Silent mode" with 0dB fan on in Asus GPU Tweak 2 util, which offers a 4% overclock. See screenshot below)
https://cdn.discordapp.com/attachments/339810774501294087/397340658085396490/unknown.png
I struggle with thermal loads in my current setup so I dont have the thermal headroom to overclock it further (yet), When I try pushing the envelope with the overclocking the card hits thermal limit and throttles back and actually becomes slower than stock so theres no point. I'm going to change my case later this month and go with an AIO water cooler for the CPU which should give the machine much more room to play with pushing the GFX card
Back to your original question, from the videocard benchmarks website (first screenshot in this post) you can see the 1080ti has a 35% hike in performance on the 1070, but its twice the price. It is also snapping at the heels of the Titan V, which costs 4 times as much as the 1080ti. And as I've demonstrated it can be brought within spitting distance of the TitanV's performance with even a mild overclock. It will come down to a personal decision on how much you want that extra 35% performance and how much you are willing to spend to get it?
My balls are big but they arent made of crystal, so so I cannot accurately foretell what new models NVIDIA will be releasing, however I'd be reticent not to point out that the titanV was pitched at and designed towards AI (aka Machine Learning or Deep Learning) so the benchmarks don't play to its strong suit. Were I you I'd look for January sales / special offers on an aftermarket cooler equipped GTX1080Ti such as one of the MSI / EVGA / Asus Strix models. But if in the summer NVIDIA release GTX1180Ti with another 30% performance on offer for the same money, please dont get angsty with me about it.