[Balr14]

Detail terrain brushes will cut down significantly on vis data and won't impact performance, since it's unlikely terrain is blocking anything from view, anyhow. LOD terrain can be used effectively, but remember each LOD section is 256 faces. Keep it as smooth as possible ( to cut down on visible faces) and keep the size managable with lots of brushes around it to give it room to cull.

Keep in mind that since detail brushes are ignored by BSP, they don't exist when you get to QRAD. Consequently, detail brushes do not have the same shadow and light properties as structural brushes. They get residual light/shadow from neighboring brushes plus receive the same ambient lighting as all entities and brushes. This may or may not bother you, just so you know it's different.

If you require other mapping assistance, my home site is the Modding Theater. I only come here every few weeks and rarely post.