Sorry, took longer time than expected to fix the 3d skybox so havent gotten around to optimising until now. I put my first bets on using func_occluders as Crazycarl suggested and that turned out ok with an fps boost of around 20.
Started adding func_occluder brushes at the spots I thought was the largest problem areas but it ended up not working very well, so I just slapped in a couple of huge occluders just to see the differance.
func_occluders in Hammer
Occluders of this size might get the server on its knees, but have to test it. They work quite well, and you don't notice much the models coming and disappearing in the line of sight. (the func_occluders are the yellow intersecting huge brushes. Trigger texture on both sides which means they occlude from both sides.)
mat_wireframe 2 on the map without occluders
As you can see all models are drawn, even those beyond the hills.
fps on the map without occluders
Not much to write home about
mat_wireframe 2 & r_visocclusion 1 on the map with occluders
Wohoo, models beyond hills are gone.
r_visocclusion 1 on the map with occluders
With r_visocclusion 1 it colors occluded models in green boxes, not occluded ones with red boxes.
My fps on the map with occluders
From 31 to 59 fps was a giant leap for my old computer
.
Intel 4 - 2.66, 1 GB ram, Geforce MX 4000.
I guess this is concidered low end today, or low medium.
Anyway, experiments will continue next with hint brushes.
Thx again for the help in the rigth direction.