Sapphire's HD 7970 Toxic is like a Lamborghini. It's expensive, not very efficient, and noisy. Unfortunately, noisy computer components don't impress people nearly as much as noisy cars, and a fast graphics card doesn't get you girls either.
Thanks to a massive overclock out of the box, the HD 7970 Toxic delivers outstanding performance results for a HD 7970. In our testing, the card ends up 11% faster than the HD 7970 reference design, 3% faster than the HD 7970 GHz Edition and 4% faster than GTX 680. If that's not enough for you, you can use the card's dual-BIOS feature to switch to an extreme BIOS called "Lethal Boost", increasing clocks and voltages even further. The Lethal Boost BIOS provides an extra 5% performance boost on top of the card's already very good performance, making the card faster than any single-GPU card we ever tested.
Sapphire has equipped their card with 6 GB of GDDR5 memory. This might look nice on paper, but we don't see any performance advantage as a result of 6 GB of memory in all of our testing. We recently tested a 4 GB GTX 680 and did some memory consumption testing, which showed that, even on a triple-30" monitor setup, the maximum memory usage does not exceed 3 GB, so buying a card with 6 GB of memory seems unnecessary.
All the extra performance comes at the price of massively increased power consumption because Sapphire not only cranked up the clocks, but also the voltages to keep the card stable. During typical gaming, we see the HD 7970 Toxic, similar to the HD 7970 GHz Edition, use 203 W of power, whereas the original HD 7970 only used 163 Watts. Once you switch the card into Lethal Boost mode, this number increases by 10% to 222 W for a 5% performance gain. When looking at performance per Watt, the card does 10% worse than the HD 7970, but a little bit better than AMD's HD 7970 GHz Edition. NVIDIA's Kepler based cards are around 20-30% more efficient.
All power that a graphics card consumes is turned into heat that has to go somewhere. The massive dual-fan cooler certainly keeps the card cool, but can only do so at high RPMs and with quite a bit of noise. While fan noise is lower than the HD 7970 with the normal BIOS, fan noise ends up higher than the HD 7970 GHz Edition once you use Lethal Boost. NVIDIA Kepler based cards offer much better low noise capabilities. It also doesn't help fan noise that Sapphire apparently favorers low temperatures over low fan noise; a more balanced approach would have been better and could have been integrated with the dual BIOS feature.
Overclocking the card worked well thanks to the increased GPU voltage. With a maximum clock of 1275 MHz, the card reaches the highest clock speeds of all HD 7970 cards we ever tested by quite a large margin. Memory overclocking works well and reaches 1885 MHz, which is the second highest we've seen.
Sapphire is asking $700 for their card, which seems very expensive when looking at performance per Dollar. The GTX 680 and HD 7970 GHz offer 35% more performance per Dollar and the GTX 670 even offers 56% more performance for your money. You could build a dual GTX 670 SLI rig with those 700 Dollars. Sapphire does include a useful bundle with the card that includes an active single-link DP to DVI adapter, making running three 1080p monitors in EyeFinity possible without any additional purchases.
If you want the absolute fastest single-GPU card, have the money and can stand the noise, you should definitely consider the Sapphire HD 7970 Toxic; other users should probably look at a GTX 680 or 670.