As it stands right now, a lot of shared Mesa code is built as a shared library (libdricore), which the independent Mesa/Gallium3D drivers link against for sharing common code paths. This is a change that was done by Canonical two years ago in having this single shared library for saving space on the Ubuntu CDs.
While it saves a few megabytes of space, having this shared Mesa library between drivers ends up requiring all of Mesa's symbols be public. All the public symbols could mean a program calling one of the Mesa symbols or even overriding one with their own code.
In clearing up the public symbol problem, Eric Anholt has been experimenting with "mega drivers" where the drivers are built together as a single .so file. The win in this case is that by building all the code together, there's new compiler optimizations that can be tapped and allowing link-time optimizations that aren't possible with the shared library approach.
Eric has stumbled across a few problems so far in the mega drivers implementation, but he's continuing to experiment with new code. In terms of the benefits making it worthwhile, Anholt says, "Initial performance results: Enabling LTO on a dricore build, I saw a -0.798709% +/- 0.333703% (n=30) effect on INTEL_NO_HW=1 cairo-gl runtime. On a megadrivers+LTO compared to non-megadrivers, non-LTO, the difference was -6.35008% +/- 0.675067% (n=10). I think this is definitely promising."
Eric says he will be talking at DebConf about this Mesa Mega Drivers design and also some ideas for how to get graphics driver updates out faster. More details can be found from his blog post.