Phoronix published a review on the The OpenGL Speed & Performance-Per-Watt From The Radeon RX 480 To HD 4850/4870

A quote from the article:

With the Radeon RX 480 Linux review now being out of the way and our various other RX 480 Linux benchmarks, the latest results I have to share with being a benchmarking fanatic are RX 480 results with high-end AMD GPU tests of each generation going back to the Radeon HD 4850/4870 (RV770) days. This article has high-end GPUs from the RX 480 to RX 200, HD 7900, HD 6900, HD 6800, HD 5800, and HD 4800 series compared side-by-side with the latest open-source Radeon Linux graphics driver code. Not only is the raw performance being looked at but the system power consumption was also being polled in real-time for looking at the performance-per-Watt too. For any other benchmarking fanatics curious about the Radeon GPU evolution over the past eight years (RV770 launch in 2008), here are the numbers to enjoy.

All of these tests were done with the latest open-source AMD Linux driver stack: Linux 4.7 Git and Mesa 12.1-dev git-6b0ac95 along with the latest user-space components (and LLVM 3.9 SVN) via the Launchpad Padoka PPA. Compared to the proprietary driver that has long abandoned older generations of GPU support, the latest open-source code continues to be maintained for all of these legacy GPUs going well before the Radeon HD 4000 series. The Radeon HD 4000 series was used as far back as testing for still providing reasonable OpenGL 3.3 support and not being too weak for comparing to modern GPUs.

 The OpenGL Speed & Performance-Per-Watt From The Radeon RX 480 To HD 4850/4870 @ Phoronix