Scene Capture Custom Projection Matrix

I want to use the custom projection matrix for scene captures because I don’t need to render the entire view. I want to clip or cull out the sides, but the comment says the custom projection matrix doesn’t affect culling which means it wouldn’t help performance either. Is this true?

Another idea I had would be to have multiple clipping planes. I assume this could be done but I would need to do a custom build of the engine? Any ideas or thoughts would be much appreciated as sceneCaptures eat up performance. Thanks.