The best way I know of to maintain a high graphics core clock rate is to increase its power limit and fan speed. I was able to do this with the GPU set to 1100 MHz and the DRAM pushed to GDDR5-5600. Of course, you have to remember that AMD's PowerTune technology starts scaling back frequency once the Hawaii GPU hits 94 degrees and fan speed tops out at factory-defined levels.
AMD tries to keep the noise of its under-built thermal solution under control by letting its GPU get fairly hot, at which point the fan spins up to around 60% duty cycle. The fan only spins faster if it detects a thermal crisis. But those settings wouldn’t let these GPUs run near their limits.
I chose a maximum fan speed of 100% at a target maximum of 80° Celsius, even though AMD would have us believe that's overkill for a GPU that can run at maximum performance at higher temperatures.
Even though my CPU's temperatures remained low enough to prevent thermal throttling, the system would lose performance anyway after several minutes under full load. I opened the case and reached inside to find a scalding hot sink on the motherboard’s voltage regulator! Placing the left-over intake fan as a top-panel intake over the voltage regulator should have worked, but the blades whizzing past the fan grille sounded like a very loud hornet’s nest. Flipping the fan over to create exhaust helped cut the noise, but also reduced airflow over the voltage regulator. Fortunately, I kept a solution in reserve.
Voltage regulator sinks are typically designed to work in conjunction with a CPU fan. But liquid cooling moves those fans away from the processor interface. Even non-traditional gaming system makers like Lenovo recognize that problem, and have addressed it. Lacking a custom-fit solution or even so much as screw channels to secure a fan upon the X79 Extreme4’s PWM cooler, Antec’s universal SpotCool fits the build.
The case’s original intake fan remains mounted as exhaust above the motherboard, even though it’s no longer required. A little extra cooling never hurt.
As a result of my aggressive GPU fan settings, maximum system noise increased from 40.1 to 54.1 decibels at one meter after overclocking, illustrating Chris' notion that these cards either run too loud, or can't be cooled amply using the reference solution to keep them at maximum speed. At least the stock setup proves my advice about choosing the right case to muffle noise from the graphics cards.