There are existing composite body parts on the current M3, the front fenders and perhaps others that are SMC (sheet molding compound). It is a fiber and binder just like CF but much less exotic fibers (not carbon) and less exotic binder. In short they are much less cheaper and much less strong. Anyway, the point is that these are already very light and handle the relatively minor aero and NVH loads quite well. You can't save much weight here. These parts are not a good choice for CF as the parts are entirely non load bearing. Doors might be a possible choice though for real CF since they are key load bearing items in side crash scenario.
A completely aluminum chassis would represent a milestone for BMW however, Audi did this a long long time ago but if I recall correctly in their first try at it did not save much weight.
I'm not following the base car closely enough but isn't it mature enough that we know what it is built from? I highly doubt there will be a different chassis between the base and M car. That is simply unaffordable