...wouldn't it be more prudent to contribute to the existing effort...?

Sure, and that's exactly what people are doing. While following what autrijus is working on, a lot of people have been able to pick up some knowledge of how Perl 6 will work. They've found difficult and ambiguous sections of documentation. They've asked questions. They've found things that didn't seem to work. They've written code and contributed test cases. All of these things help to improve Perl 6 and will help to shorten timeline to get to Perl 6.