Marek published a patch this weekend on the DRI development list for the Radeon DRM Driver that fixes the render back-end setup for the Southern Islands and Sea Islands graphics processors (Radeon HD 7000 series and newer).
It turns out the open-source AMD Radeon graphics driver on kernels up through Linux 3.13 only enables the render back-ends for the first shader engine and disables the rest by accident. With Marek's correction with the new patch that changes less than a dozen lines of code, the other render back-ends are enabled and it sharply improves the performance.
In one of the sample results shown by Marek, this small patch goes from having Unigine Sanctuary running at 15 FPS on the modern RadeonSI stack to now running at 90 FPS! Fan noise will be up though now since the Southern/Sea Islands graphics cards can now reach full load.
More details on this new patch that will hopefully be merged for the Linux 3.14 kernel can be found on the dri-devel list.