Line of Sight Between Z-Levels & Stability

I found a nice three-story house to fortify in my newest playthrough. Can I open the curtains on second or third story windows to let in the light without zombies on the ground seeing me? If they can see me will they navigate their way upstairs to ruin my day, or will they just wander around on the ground floor?

I also might as well ask about Z-Level worlds in general. I’ve only played a couple games with them, but they seems to work without a hitch from what I’ve seen. Anyone run into any game-breaking or save-borking problems with them? I’ve read that they slow the game down, does it get worse with every new Z-level area you visit? I have a fairly nice processor (i5-4690k @ 3.5ghz), would I notice a difference?

All z-level calculations combined are less expensive than drawing calls alone, but they can be noticeable in some specific cases.