Unladen Swallow does support Python C modules. All the other implementations (IronPython, JPython and PyPy) don't and they are not decidingly faster than CPython. The developers from Google looked at JPython but the missing C module support was a major reason not to follow that road.
It's certainly super-smart to adress the compatibility issue by starting with the CPython source code. It has also the nice effect that patches are merged upstream. The next CPython releases will have improved performance based on work done in the project.
Whether the LLVM implementation will really lead to the performance targets set by the project is an open question. A concern is also that LLVM requires a C++ compiler making support on exotic platforms more difficult.
Regardless of those concerns Unladen Swallow is a project worth being started.