[...] 2/ As we have previously discussed, I also like the idea of letting the developer decide on which parts he/she wants strict/dynamic typing. And if this is the only option to get a huge performance improvement for the 1.5 branch then I guess it may be worth to offer this option to our users.

Concluding, for me the near future of Groovy depends a lot of the language consistency and performance (and in a smaller measure to its API stability -- remember I've said near future). I would like to have a new version whose overall performance is improved thanks to clarifying the details of the MOP/Categories implementation. And I do believe that thanks to the consistency introduced by such a reimplementation most of the people will be willing to switch to the new version. However, for those that have invested a lot in the current version, the solution you're suggesting may work (and I do think it may be useful also in the next version).