Opaque windows give a mild performance boost as it hides what is behind, which allows the occlusion manager to not render some of what is behind. Chunks layers are 16x16x16m and occluded as a whole, so depending on where the window lies in chunk space, it will still be drawing closer things behind the window as they are in the same chunk with the window.
AO is not a performance boost. It it a shadowing technique that darkens ambient light in angles/corners to mimic how they tend to look.
Yes. We will try.
We are regularly cleaning things up, but it takes time and time is short. Sometimes we don't even know if something is used because the original programmer may no longer be around, so then you have to track the data use.