Not sure about how high Max Voltages on Stock Air or even Water Cooled GTX 980 Classifieds will handle, But if You Don't Care about any EVGA Warranties, TiN at Kingpincooling.com stated this about 980 GPU Vcore Limits using Extreme Hardware Mods under LN2 Cooling:
"You can go as high as 1.7-1.85V with this kind of modification, so it's suitable
for extreme overclocking.
But beware, safe voltages even on LN2 are rather
around 1.5-1.6ish range, which is still enough to get decent overclocking on
most of the cards. 1700-1800MHz clockspeeds should not be an issue."
http://kingpincooling.com/forum/showthread.php?t=2886 Just Choose Your Poison and Take Your Chances if You want to.....