I just noticed a detail and wanted to say it.
Anyone that uses clip brushes for the collision of detailed static actors placed on top of a solid ground brush should have in mind that the clip brushes cast (dynamic) shadows on the solid brush underneath them,
unless you make these clip brushes a world model.
The good thing is that the clip brushes do not receive stencil shadows,
(this way the invisible brushes do not have to be the exact same shape as the actor there used to,and you can have a larger lower poly clip model for a staticentity rock-which anyway would project from the highpoly at some areas.
Also clip walls can restrict flying pawns from getting out of the game area without casting shadow on the invisible barrier.
I say it because when you use dynamic lights you must have in mind that all these invisible clip brushes will be added to the dynamic shadow calculation and drop the framerate anyway,so whatever collision brush you dont need to cast shadows,add it as a world model (no animation needed.