Read that earlier article for more details. Since then, Phoronix readers were quick to point out that HyperZ is now disabled by default for the R600 and RadeonSI Gallium3D drivers. HyperZ is known to provide significant performance improvements but is now disabled by default over apparent issues with some Linux games on some hardware. Now to have HyperZ and its benefits you will need to set the R600_DEBUG=hyperz environment variable.
However, this doesn't appear to be the only cause for the performance drop when upgrading Ubuntu 14.04's graphics stack. Other regressions have also been reported by other users of the open-source AMD driver. "This is not completely from HyperZ and so still worth investigating. We have reports from Luke and dungeon on this forum that confirm there's another regression besides the intended hyperz change. Luke specifically tested it, dungeon's case is media apps that do not use the Z buffer." There's more comments in this forum post about the issue unrelated to HyperZ.
Anyhow, on an older AMD Opteron system with Radeon HD 4870 graphics I am in the process of testing every Ubuntu Linux releases where there's hardware support available and ultimately going back far enough to test the legacy Catalyst driver that supports the Radeon HD 4870 "RV770" graphics card. This article should provide a nice look at how the open-source performance has evolved and how it's running against the legacy Catalyst driver. Yesterday though I ran a similar Ubuntu 14.04 Linux 3.13 + Mesa 10.1 vs. Linux 3.14 + Mesa 10.2 graphics benchmark on this Opteron 2384 + Radeon HD 4870 system and found similar results.
For those curious about those extra results from yesterday, you can find them on OpenBenchmarking.org via 1403016-PL-UBUNTUTES16.