Well, I think there might be conflation of terminology here. The Geometry tool does give the option for Solid, Semi-Solid and Non-Solid; plus it gives additive and subtractive options. Then you got (which I *think *is where the conflation is) the option to make a box/cube and cylinder shape ‘hollow’ in the options.
So, the poster is (again, I think) asking whether if making “…some buildings that are just there for show…” hollow, thereby increasing the total amount of geometry by 6 faces per building, makes any difference to performance.
My answer is that, BSP is not to be used for final game, but if you have something which is just for show, like a row of buildings; then test with BSP, but convert to static mesh/FBX and use optimisation in-engine to figure out what the ‘hit’ is to your frames.