Sounds like a good idea if the releases can then also be staggered (but obviously tested in tandem).
I would recommend strongly AGAINST having a "convenience" git submodule arrangement as it means you easily get wondering commit states in different checkouts, similar to what easily happened with svn.
A daily SNAPSHOT build system means Devs can work on just one of the repositories at a time, without having to git pull the others to stay up to date.
Obviously if someone is planning breaking changes that affect multiple repos they would have to build both every day (or change their local snapshot policy), but that is not different than today's situation with a single repo.
Have you tested that the proposed structure don't introduce any circular dependencies between the (modules of) the new git repos?
