The Map will be getting to Big if we create a Mountains where you have to go down. ( in a smaller Map you just need to do stravejumps, q3 movements, to get a Speed like hell). but we are in the right Way.
You can achieve open map using far-clipping, so you need less vis-blocking to save framerate. This apply only for the renderer, the physical engine still have to perform collision test on solid object that are currently hidden.