Of course, these tests were just from a Mobility Radeon X300 that was available, but the story is likely the same across the R300 spectrum. With the R500 and newer, at least that's where the performance is becoming more competitive with the open-source Linux driver compared to Catalyst, as many earlier Phoronix tests have shown. However, in most tests it is still far from performance or feature parity. The open-source driver has come along way since 2009 when the Catalyst R300-R500 support was dropped, but it still can be sluggish and not running as well as the old blobs. Also worse off on modern Linux than using distributions and drivers from years ago is the power consumption on vintage hardware. Below is a look at the power consumption when idling with the ThinkPad R52 under Ubuntu 6.06.1 LTS, Ubuntu 8.10, and Ubuntu 11.10 with the aforementioned driver configurations.
Ubuntu 11.10 is consuming 22% more power than Ubuntu 8.10! This is on the ThinkPad R52 that has been running "out of the box" under Linux for more than a half-decade.
When running the Warsow game and measuring the battery power consumption of the Intel Pentium M + ATI Mobility Radeon X300 notebook, its power consumption was higher with the Catalyst driver under Ubuntu 8.10. However, in that configuration is where the frame-rate in Warsow was 2.5x higher. The Ubuntu 8.10 power consumption with the faster performance had a premium of +12% power consumption. Ubuntu 6.06.1 LTS meanwhile was consuming less power than Ubuntu 8.10 and Ubuntu 11.10 -- the Oneiric Ocelot was going through just over 10% more power than the six-year-old LTS distribution. Of course, the graphics driver is far from being the only change in the distribution and the power consumption may be attributed in large part due to kernel and user-space differences. (There's also some outstanding Linux power regressions that I'm aware of that will still be shared soon.) For the end-user though that does not matter and all they will see is the power consumption going up and a lower battery life.
What am I trying to get after with these results? Making Linux users aware who are still running vintage hardware that it is always not best to upgrade your distribution. The ancient Radeon GPU with the open-source Linux driver is still behind the Catalyst driver and is likely to never catch up. The R300 Gallium3D driver is mostly in maintenance mode these days with just the occasional contribution from a community developer or R500-specific work going into this driver. The R300 Gallium3D driver already has 2D color tiling support, Hyper-Z, and implementations of various other features that the hardware supports, but it is still not up to Catalyst levels nor to the same level of dynamic power management. There's also the OpenGL rendering bugs that were illustrated in this article.
I am also not trying to say AMD should invest any further resources in bettering the R300 Linux support. This commodity hardware is more than a half-decade old and the old Linux distributions and drivers do remain easily obtainable. Users can still fetch these old components and selectively build newer pieces of software for their stack at their own risk and time should they choose to not upgrade their hardware. AMD still has plenty of work to do for their modern Radeon hardware and is a for-profit organization that has obligations to their shareholders. The R300 code and documentation is available for any independent users/developers with an active interest in furthering the support. I've made it clear already that I feel Linux support for old hardware should be dropped from newer releases rather than just shipping this half-baked support that's suffering bit-rot.
At least with the R500 generations and newer, this is where AMD's open-source strategy has really taken shape and -- as already said -- for some Radeon graphics processors the Gallium3D driver stack is more competitive with Catalyst. The AMD open-source strategy wasn't announced until after R600-class hardware was already shipping and really intended for the support was R500/R600 series and newer. The open-source R300 support did come before that through reverse-engineering and minor contributions from ATI, and then long afterwards some improvements from AMD came once the open-source strategy was conceived.
By the time the next round of hardware support from their Catalyst driver, hopefully the open-source Radeon stack at that point for R600+ will be in great shape and be much better than when the R300-R500 coverage was dropped in 2009. To see where the support is at for modern Radeon hardware, read Radeon Gallium3D With Mesa 8.0: Goes Up & Down and The Most Comprehensive AMD Radeon Linux Graphics Comparison, among many other Phoronix graphics articles.