I think it's way too early to discuss removing FineGrainedAtomicMap and AtomicMap, as long as we don't have a concrete alternative with similar properties.
Cache.getGroup(groupName) is just a method name at this point, we don't have any idea how it will compare to AtomicMap/FineGrainedAtomicMap from a transaction isolation or performance perspective. BTW, do we really need the group name to be a String?
A good way to prove that the grouping API is a proper replacement for the atomic maps would be to replace the usage of atomic maps in the Tree module with the grouping API. Unless we plan to drop the Tree module completely...