Quake3World

I'll probably post a screenie for that if anyone asks for it because my explanation is too abstract for him. But for now, I just wanted to ask if it makes more sense (regarding compiling time and in-game FPS and so on, say: performance) to have the downside of your own map completely flat and even OR if it's okay to have an uneven surface resulting from roads and sidewalks having different heights.
In detail: the roads consist of brushes being 8 units high, the pavements are also 8 units high but start where the road brushes end to have the mentioned effect that roads and pavements are visually separated if you play the map. So if you look at the bottom side of the map there are obviously these spaces created by the pavements. So the question is: is it better to stretch the pavement brushes from 8 to 16 units thus resulting in a flat bottom side and avoiding additional (obviously unnecessary) tiny clusters that way; or is it more of a problem to have many orignally 8-brushes turned into bigger 16-brushes?

Depends if you're talking about detail or structural brushes. Also depends on what you have caulked.

Assuming everything is detail brushes, and the bottom and sides of the brushes are properly caulked, there will be no performance difference whether or not the bottom faces of the brushes are flush.

Structural brushes won't make a difference on the bottom side since faces outside of the map (not convex) are ignored by -vis. If properly caulked, they will be ignored by -bsp and -light.

General rule of thumb:
1) Caulk, caulk, caulk
2) Convert small brushes to detail, use caulk-hull construction method.