Yeah, it's a great way to optimize and keep a tall skybox, but you gotta be careful.
I didn't notice until after final compile on Orient_b3 (and wasn't gonna recompile), but the tops of the temples are in 'boxes' that are taller than the rest of the sky to keep vis_leafs/vision to a minimum.
The box is in real tight around the top roof. Unfortunately I ended up with a little leaf up in the top and if you stand in one doorway and look at the blue temple that leaf doesn't draw and part of the roof clips out.
It's not very noticeable and I don't think anyone has seen it yet but it's there.
Once I add a func_viscluster it'll be fine, but that's the type of thing you gotta watch for.