chromatic says a factor of 2.0 is a worthwhile target (http://lwn.net/Articles/398051/). I think that's the best you'll get since good developers—especially perl6 developers—generally are copiously optimistic.
We haven't reached the point of dramatic optimizations yet. We're making some architectural changes in Parrot (the Lorito project) to allow for tracing JIT, bytecode optimizations, and slimmer deployment of dependency-traced applications.