Running The Oibaf PPA On Ubuntu 14.10
Mesa 10.3 will hopefully still make it into Ubuntu 14.10 ahead of its debut next month since Mesa 10.3 brings many new features to the commonly used open-source Intel, Radeon, and Nouveau graphics drivers (along with promising drivers like Freedreno and VC4). If you want to try running the newest open-source user-space graphics driver code on Ubuntu 14.10, it can be easily achieved today using the well known Oibaf PPA.
Ubuntu 14.10 has been supported by the Oibaf graphics driver Personal Package Archive for a few weeks already so it's just a matter of running sudo apt-add-repository ppa:oibaf/graphics-drivers && sudo apt-get update && sudo apt-get dist-upgrade followed by a reboot. The Oibaf graphics drivers for Ubuntu 14.04 LTS and 14.10 continue to follow the latest Mesa 10.4-devel code and are updated from Git almost daily. More details on the PPA's latest state can be found via Launchpad.net.
Lastly on a minor note, I did run some fresh Radeon HD 6870 R600 Gallium3D benchmarks between Ubuntu 14.10 with its Linux 3.16 kernel on Mesa 10.2.6 as found right now in the Utopic Unicorn archive against Mesa 10.4-devel from the Oibaf PPA, but there's few performance changes to note. Aside from the newer 10.3~10.4 features, better performance can be expected out of RadeonSI Gallium3D driver, Nouveau, and R600g in select cases.