I would take a step back and rationalize the fact the more voltage isn't always better.
Currently running 1.213mV @ 1412mhz & 55*c
I think voltage is more sustainable with the DIGI+ VRM & Black Metallic Capacitors.
Sure other cards maybe easier to unlock and force 1.5v and really go at the bleeding edge. I know the Matrix is one LN2 Jumper away from being unlocked. And if your seeking that 1500+mhz threshold that's what I would suggest. However I would argue that on a 28nm chip running 1.213v.
I don't expect the Poseidon 980Ti release to take to long, it's already confirmed by Asus that they will be making the 980Ti model in the Poseidon series (Asus FTW).
I haven't felt the need to push this card further then what GPUTweak will let me with the stock bios, but it is certainly possible. There are bios that will unlock the mV and let you go up to 1.5v in GPUTweak.
ROG2015 Back in Red&Black
NZXT.H440 ~ M7F ~ i7-4790k ~ Poseidon GTX980 ~ G-Skill 2400 16gb ~ 850Pro 256gb ~ ThermalTake 1050w DPS ~ VG248QE