Hi Puma,
I'd say that the "architectural massing" is already fine (although there must be some structures under the seats to support those slanting areas).
Now you'd definitely need to add
some (even) better textures (from a distance, this current texture is tiling badly as well as too "uniform" for the building)
some modelling/texturing details (like around the arches - they obviously need to be supported somehow)
if you make sect5ions, yes, definitely use TIG's SectionCutFace plugin so that the massing doesn't look "hollow"
Otherwise I love the ones when you put it into the environment - that's very "professional" looking (although I don't really like this term itself).
Please, carry on developing the project.