While there are a lot of talks about BSP tools and better CSG workflow, no one seem to remember that all older engines that use BSP/CSG have special “nodraw” material which tells map compiler or renderer to discard or not to render triangles with “nodraw” material. This effectively discards all the back faces in any geometry reducing rendering overhead.
Here is a mock-up showing such approach:
Does UE4 have something like that? (perhaps default material with some option checked to tell renderer not to render those triangles)
P.S. Note that it’s not an “invisible” material, where triangles you processed, but you don’t see them in the scene (invisible material would still have triangles in wireframe mode, while “nodraw” material wouldn’t have any)