It's true nowadays it is not necessary to optimize a goldsrc map as could it be 10 years ago, but I'm wondering the way that vis works in the game.
I disagree, some players are still running on Windows XP with 1 core CPU and a few hundred of Mb of RAM and GPU that barely support "modern shaders" and OpenGL versions (3.X, 4.X), it's not my case, but I know some people who does (and they aren't lying, they sent me screenshots of their report with HWINFO32).
Even with fast computers, optimizing to the max can save precious FPS.
"People say that you should not micro-optimize. But if what you love is micro-optimization... that's what you should do." - Linus Torvalds (father of Linux), (2012-06-14). Audience Q&A following interview panel at Aalto University Center.
And I think this is true as well for any aspect of game development and not only programming (level design, art, modeling...)
Rendering is not based upon line of sight. Roughly, if the area the player is in can see the geometry it will render.
To be more precise, it's the PVS (Potential Visibility Set).
Please note that my explanations may be incorrect and anyone is free to correct me if I'm wrong (I'm not very good at explaining stuff so I'll try to do my best)
In all maps there is a BSP tree and every "leaf" of that tree represent a "collection of polygons" or an "area" in your map if you prefer. In other words, all those BSP leaves form a single BSP tree which is the map itself.
Every in-game frame, the engine will determine on which BSP leaf you are currently in and calculate which leaves you can actually see and those you may be able to see. Once that process is done, the rendering is done. Bruce mentioned it in his post and I'm gonna post it again for the record : entities are treated differently.
Something you can do is try to help the compilers delimit those leaves thanks to SKIP and HINT brushes.
This excellent tutorial will explain you how it works.
A word of warning : don't abuse HINT/SKIP brushes and use it intelligently otherwise you will have the exact opposite effect of optimization, they are here to help but not replace the job of the compilers.
In the case of your map, I would try to cover the warehouse door on the left, the back door on the right and the ventilation shaft entrance at the rooftop (from the level of the roof to the "ceiling" of the shaft). This will separate the "inner" and "outer" parts of your map.
UPDATE :
Another good tutorial by Andy posted here on TWHL about HINT/SKIP