The reason 3rd party heatsinks haven't actually improved much is because of GPU Boost 2.0. In my understanding that since it's temperature based it is actually more aggressive than 1.0 and tries to push the GPU's clock and voltages more if the temperatures allow it. Basically from what I've read it simply pushes the GPU to its maximum allowed boost clock and voltage at an attempt to get to 80C and then it attempts to regulate the clock speed and voltage to keep it as close to 80C as possible.
Possibly if you look through this thread here you may be able to find a vbios for your Gigabytes that has GPU Boost 2.0 and thermal throttling disabled. Could prove to increase overall performance too since it'll be at a fixed clock speed.