While the model optimizations you speak of are basic and always needed. You can also further benefit from using the regions to block-clip whole areas. Even if inside a building, if the far 50% is full none-visible to the player, then clip it out with a region.

In the screen shot above there are clearly rooms that are none visible to the current view position. I believe--> Using regions skips the step of the engine deciding if a area should be culled. Thus speeding the render.