It would be sad if existing performance improvements from Unladen Swallow based on Python 2.x would not go into eg. a python 2.7 release. Especially as a lot of python2 applications may be around for a long time.
If marketing reasons would bring it to only python 3.x, I guess the sentiment towards python in general could backfire in the community. Nobody likes to be forced (even not gently) if not needed.