Since you will be rolling this out network wide and you've said:
We feel that in order to properly evaluate the changes to presentation, language and functionality, the changes need to be seen in the context of the site, by users who encounter them through normal site usage.
Will you be doing a similar test period for sites other than Stack Overflow? For example, the automatic comments for duplicates ("Does this answer your question? [link]") doesn't really make much sense on Code-Golf due to it not being a standard Q&A site. There's also been talk about how the phrasing of some notices would be inaccurate on other sites.
If you aren't, will sites at least be able to edit the phrasing of notices that don't make as much sense in the context of their sites?