Up to current date a 6 GB card, at least for 1080P is still sufficient on VRAM for as good as any game i know of (4 GB can already become a slight issue). 2160P/4K can be more tricky but i suspect this card as a single configuration is still not powerful enough in order to run a 2160P/4K single-chipped. I think the 1080 TI TFLOP value is about 12 TFLOPS (Vega appears to be in this spot aswell). It could even beat the current Titan X Pascal, who knows.
Of course, the core amount is most likely reduced on the TI vs. Titan still but naturally a reduced core count means there is higher clocks possible (lesser instability issues), reason why a TI is in many cases matching or even beating a Titan.
However, i do put lot of trust on the next TI card in line, it may include about 18 TFLOPS (16 TFLOPS is my target for a sufficient single 2160P/4K card) and this is finally a sufficient card. So the next card i´m gonna upgrade is a 2080 TI or however Nvidia may be calling it but this will take more than a year for sure, so i will be very fine using my 980 TI for the next 1+ years, maybe even 2 years. A good OC 980 TI (1400 MHz+) is about 8 TFLOPS in comparison and for 1080P still a powerful card. Of course a GTX 1080 (non TI) at close to 2000 MHz OC may end at around 10-11 TFLOPS and beat the 980 TI by about 20% increased performance. GTX 1080 TI probably at max OC even going up to 13-14 TFLOPs i assume, but still not a sufficient single 2160P/4K card.
AMD unfortunately is pretty much asleep for long time already but i hope they gonna compete another time...
So far, Nvidia didn´t feel the need to use a fully unlocked Pascal on a consumer card, not even the mighty Titan X is fully unlocked but who cares as long as there is no competition... sad bear story.