Regarding 980 Ti overclocks, I will share my personal experience with three different cards.
Card 1 (Single Card)- Boosted to about 1480mhz under air w/ minor voltage increase, good temps. I couldn't get 1500mhz out of it. I got around 8.2ghz on the memory, good for around 393GB/s of memory bandwidth.
I sold that first card and got two different ones for SLI
Card 2 & 3 (SLI) - Boosts to about 1470~mhz under air w/ SLI on stock voltage. I didn't increase voltage since I don't want to cook the upper card with crazy temps. No reason to push to the max since in SLI they can play just about anything with every setting anyway. I didn't bother to test OCs separately or with increased voltage, so I dunno how much higher it can go. Maybe 1500mhz, maybe not. Memory is good for 8.2ghz for both cards, good for around 393GB/s of memory bandwidth.
So out of my three cards, none of them hit 1500mhz. One had minor voltage bump, two had no voltage and in SLI so I was a bit conservative. But all three were really close.