Silicon lottery and if it's a good card(like KPE or Matrix), it can go up to 1500MHz. Even 1600-1700MHz in some extreme situations. But usually 1300MHz is the most you will be able to get.
I managed to get 1520MHz with 1.230 volt.
It was around 72C, fans running at full speed.
My card gets 1340MHz max boost speed by default, and stays there without throttling, so I think 1400MHz should be quite possible on most cards.
Custom PCB and all that power phase jazz is good to have but it all comes down to silicon lottery in the end.
I got a reference EVGA and it overclocks to 1520~ MHz on Air(you can see mine in valley benchmark thread). I'm not even using a modded Bios yet, I'm thinking about watercooling this bad boy too, pretty sure I can go higher clock speeds with watercooling. If I were you I would just get a reference 980Ti.
I told you I would come back with a Titan or 980Ti, finally got rid of that 280X.
Been waiting for this since September.
GPU was running at constant 1507MHz btw, according to Afterburner.
78.7 ASIC Quality.
Reference design EVGA card.