God, I'm a total sucker for a project with a good bibliography. Must resist urge to go to grad school.
They seem to actually have a solid (early) release, and it does demonstrate some impressive speed improvements.
They do have one test for Psyco, but unfortunately results are not included in the wiki (Spitfire, which is Psycoed version of SlowSpitfire).
From my experience, Psyco performs very well exactly in situations where also they got more significant speedups (like pickling/unpickling).
PyPy has an LLVM backend: http://codespeak.net/pypy/dist/pypy/doc/home.html
For charting the long-term performance trend of the project, Unladen Swallow makes use of Google's standard internal performance measurement framework. Project members will post regular performance updates to the mailing lists.
Besides that, from a practical standpoint I'd guess the Unladen-Swallow team wants a codebase on more solid footing, like the mature 2.6.x series, as opposed to Python 3.x which is in a greater state of flux.
Python 2.6.1 (r261:67517, Dec 4 2008, 16:51:00) [MSC v.1500 32 bit (Intel)] on
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy
>>> numpy.array( [1, 2, 3] )
array([1, 2, 3])
That said, I'd like to take a moment to express my frustration with installing or compiling PyGtkGLExt ( http://gtkglext.sourceforge.net/ ) on Windows with Python 2.6. The only installers I've found for PyGtkGLExt are for Python 2.4 ( http://www.stephanebrunet.net/gtkglext/pygtkglext-1.1.0.win3... ) and Python 2.5 ( http://www.stephanebrunet.net/gtkglext/pygtkglext-1.1.0.win3... ). Building it from source seems like a daunting task. Various build system utilities have to be installed first, such as MingW + MSYS, and pkg-config (which seems incredibly difficult to set up correctly on Windows). Also, it appears that the default maketools installed with MingW are too out-of-date to be used to build it. So far, upgrading MingW's autoconf / m4 / other tools is proving to be difficult at best. So at this point I'm planning to downgrade from Python 2.6 to 2.5 to save time, since it's basically impossible for non-ninjas to install or build PyGtkGLExt on Windows. I guess sticking with older versions is sometimes beneficial...
To hedge a little, I say all this from a distance: I don't use Python on a daily -- or even monthly -- basis.
In case anyone missed the reference: http://www.armory.com/swallowscenes.html