I had the 7970 VAPOR-X GHz edition until yesterday which i returned it because it was producing an annoying coil whine and also could not overclock past 1080Mhz stable (i will explain what i mean by stable)
Let me share with you my whole experience with this card because i have been testing it everyday for one month, so i know it by heart .. lol
First of all, the card looks decent and its cooling solution is the best thing it has, i could play Crysis 3 or BF3 on 3 Monitors 4K resolution and temps stay below 70c while fans are around 60% which is really quite for the performance i was getting.
The card was voltage UNLOCKED BUT and that is a big BUT, overclocking is not only about voltage flexibility, for me it was the silicon lottery and the poor quality ill cooled VRM on this card that were stopping me from reaching above 1080mhz.
Now here is what happens, the card comes factory clocked at 1050Mhz with a factory volt of 1.256v .. now via Trixx utility you can up the volt until 1.3 but that does not do any good.
Here is why, 1st of all even though people claim to reach stable clocks of above 1200Mhz, that is for the card i had is unrealistic, what people call stable is that they can run 3DMark or BF3 for few hours without the machine crashing and this is what they call stable, for me stable means that the card can run without producing any Artifacts at all by means of an Artifacts scanner, the one i used is EVGA OC Scanner which is awesome, i set the software to stop stress testing after it detects ONE or more Artifacts and this software stresses the crap out of the card, it is the most demanding stress test that puts stress on the card like 1.5X what the most demanding game does.
Anyways at 1080Mhz the card starts producing Artifacts no matter what voltage or power you give it, also above 1100Mhz the card gets really hot and the fans cross the 70% speed threshold which makes it annoyingly loud, also the VAPOR-X cooling solution does not cover the VRM and that makes it get really hot, it gets too close to 100c degree.
Also the card can not hold its 1050Mhz clock speed and clocks itself down to 1000mhz when running these demanding tests,.
Not to mention that the card produces annoying coil whine that Sapphire consider normal and not grounds for RMA
Long story short, even with unlocked voltage, in my case overclocking was a hopeless case, due to poor silicon quality and very poor VRM quality and cooling that you would hit before you hit your voltage limits.
For me i really hated this card .. and i am talking about the GHz edition here, if you happen to buy it and your experience is different please let me know.
I plan to get the Gigabyte card instead as i feel that Gigabyte is in general a better company (even though Gigabyte cards are voltage locked, as indicated above voltage unlocking did not do me any good in the Sapphire's case) plus they give 3 years warranty. but i will wait anyway for the Rumored GTX 780 due two weeks from now to see if the market changes enough for me to change my mind.
Hope i helped.