I have one more question. Would entites be culled even if I turn visibility calculation to off? I'm using isometric camera pointing at ground and not using level geometry to "hide" anything so I guess that I don't need that? The entities that are outside camera frustum should be culled anyway, right?
I'm asking because I'm considering using level geometry as a sort of bounding boxes for models (and using old colission system to save some performance) and that would generate too many unnecessary portals and make visibility calculation impossible.