Actually the voltages I listed are very realistic for HD7970 cards. Did you take a look at average overclocks on air for cream de la crop HD7970 range?
Here is a Asus Direct CU II TOP - One of the most stringent bins of HD7970's in the industry:
The Makings Of ASUS TOP Graphics Cards.
"The labs then use the selected GPUs to assemble complete graphics cards, with PCBs and memory. The products are put under high resolution automated optical inspection (AOI) hardware for an extended certification stage that ferrets out any defects or flaws in manufacturing (special attention given to soldering and contact points, such as data interfaces)."
That highly-binned 7970 chip needed
1.3V to reach 1250mhz on air. We are talking cream of the crop 7970 chips. At 1150mhz, HD7970 can't beat GTX680 and costs more. That means you are really getting into insane overclocking gambling trying to find a chip that'll do 1250mhz easily or have to pay $580-600 for non-reference 7970 that have been specifically binned (XFX Double D, Asus Direct CU, MSI Lightning).
vs. Reference GTX680 with no chip binning, no after market cooling, no beefed up components, for less $?