The majority of the Hint’ing work in GoldRush was done by hand by djbob, he’s the in house wearer of the daddy pants when it comes to such matters. 
As for when to use detail and when to use structural brushes and in the absence of Sock (as he’s on holiday) I will elaborate a little myself:
As evillair has already pointed out Structural brushes block VIS, obviously this is important to keep frame rate down and so you aren’t seeing more of the map at a certain point then you should be seeing. “r_showtris 1” is your friend when it comes to working out where the problem areas are in your map, as it’ll show you exactly what the engine is drawing from your current point of view. If you are seeing parts that you shouldn’t be able to see, then you need to invest in using some structural brushes in-between to block the view of the area.
It’s very good practice to build the core of your level in structural caulk brushes, and then slap detail brushes on top of them kind of like a skin. Everything you know won’t block VIS you really want to make as detail brushes.
Why do you not want to just build everything out of structural brushes? you may ask, because your map will take ages to compile.
Working this way will mean you’ll be able to stop the engine drawing polygons that no one is going to see in the position they are in, allowing you to add more brushwork to the current area than you’d normally be able to because of low FPS.
I hope I wasn’t repeating what you already know.
-Fluffy_gIMp