While it indeed is marked as 5.7-dev, which is because it is forked from master branch which is always marked next minor version from current release one (5.6), it is very likely that it would be next major version. The changes in this refactoring are extensive and deep, and more are probably to follow. Pretty much every extension will need to be ported. This is what major versions are for.

To my knowledge a JIT compiler is not being added to PHP 5.7. Can you clarify your comment or provide a link to the source where you heard this?