Yes, we were told we could stay to watch the fireworks when we had 9PM balcony seating for Fantasmic in Dec. Of course that assumes the fireworks are being showed after Fantasmic the date that you have Fantasmic seating.
I thought the view was great, but it could depend upon your seats. The seats to one side are less desireable for seeing the fireworks. Now you can't see the projections. and you don't have the full effect of the fireworks around you that you would have being in the hub or main street area.