Our sources that are currently preparing for AMD HD 6790 launch have confirmed that the card is a great overclocker and that disabled shaders might even be unlockable.
As you already know, the HD 6790 is based on quite well known Barts GPU, the same one behind both cards in the HD 6800 series, and it even uses the PCB that is similar, if not the same, as the one used for the HD 6870 cards. The choice of HD 6870 PCB should explain the maximum value TDP of 150W and even the fact that it needs two 6-pin PCI-Express power connectors.
For now we can only confirm that the upcoming HD 6790 cards will have an insane overclocking potential, and we are hearing that 1GHz is possible and quite easily achievable. Bear in mind that partners will surely go for their own cooling solutions that should make those overclocking ventures even easier, and if the shader unlockability gets confirmed, AMD might have a really good card on shelves once it launch on 5th of April.
We'll keep our ear close to ground to hear if any of our sources manage to confirm the shader unlock rumour.