Combine Actors to Static Mesh Vs Merge Actors

hey tim, i just want to ask some quick questions about this merge actors:

  1. What does bake vertex data to mesh ? enabling this increase performance ?

  2. Is Landscape Culling calculated just for once or it changes dynamicly according to vertices that if they can be seen or unseen ? For example you have merged a tall building but bottoms are not visible due to fog. I suppose landscape culling activates at this moment. But if you drag the building upward and bottoms become visible, landscape culling is not working because all vertices are visible ? am i correct ?