if the rumored specs are correct 2560 cuda cores 8 billion transistors and even at 10GBPS GDDR5X that will also be able to hit around the 1700mhz core clock with an overclock (tesla gp100 boosts to 1400+ stock and these chips always run slower than geforce cards so a stock 1080 should be hitting 1500+ on the core clock. Now lets compare
780 Ti vs 980 stock (big chip vs small chip)
2880 cuda cores vs 2048 cuda cores 40% favoring the 780 Ti
928mhz boost typically around 1ghz vs 1200mhz on the 980 20% favoring the 980
336gb/s bandwidth vs 224 gb/s on the 980 50% favoring the 780 Ti
3gb Vram vs 4gb Vram 25% favoring the 980
7.1 billion transistors vs 5.2 billion 40% favoring the 780 Ti
As you can see on paper the 780 Ti has a huge advantage over the 980 though is still around 15% slower compared to the 980 with driver improvements.
No if we look at rumored specifications of the 1080 lets do a 980 Ti vs 1080 specs with the current rumors on specs
980 Ti vs 1080 stock (big chip vs small chip)
2816 cuda cores vs 2560 only 10% favoring the 980 Ti vs 40% with the 780 Ti and 980
1202mhz boost clock vs 1500mhz+ favoring the 1080 25% vs 20% with the 980 and 780
336 gb/s vs 320/384gb/s bandwidth (10/12gb/s) so at worst 5% favoring the 980 Ti OR 15% favoring the 1080 vs 50% favoring the 780 Ti against the 980
6gb Vram vs 8gb Vram 2GB difference favoring the 1080 vs 1GB with the 980 and 780 Ti
8 billion transistors vs 8 billion transistors = tie vs the 780 Ti having around 40% more than the 980.
so what can you take away from this? obviously with the 980 Ti vs 1080 this is based on rumors and what it COULD look like, the point is on paper the 980 fairs a lot worse against the 780 Ti than what the 1080 does against the 980 Ti yet the 980 is still around 15% faster, so with that said, if these chips with adequate cooling could clock close to 2ghz then we could definitely be seeing a 30-40% performance increase the the p100 being about 80-90% of an increase which would be 40-50% faster than 1080. watch this space and hopefully come friday we'll be able to hear some good news