"In some ways, Unladen Swallow is more ambitious than these other projects"
Perhaps considering the aggressive road map, as you mention, but there is no Python interpreter project more ambitious than PyPy, which is one of the projects you make this comparison with. Unladen Swallow is an incremental improvement project. PyPy is a conceptual rethink giant leap forward style project. Unladen Swallow is therefore far less risky, but I'd also call PyPy rather more ambitious in comparison.