"So if I make every other brush a func_detail I'll have my shadows and lower my w_poly theoretically."
That's not right. func_detail still cuts other func_details. It does not make sense to make everything func_detail. Sometimes it makes more sense to not use func_detail because the cutting of a world brush creates a leaf portal. Excessive use of func_detail will increase w_poly count.
If you make ALL the world brushes in your map to func_details of the same detail level you won't notice any difference to world geometry. So only turn brushes to func_detail if it really is detail.
"Since it blocks vis and it would make sense to, for example, make the walls of a building func_detail while the ground is a world brush thus eliminating cutting there."
Bad idea. The func_detail blocks vis, but the floor has to be in one VIS-leaf. So for the engine the objects on the floor are in the same VIS-leaf as you and renders them though you can not see them.
"The e_poly from my experience has always been the main influence on lag online."
w_poly is world geometry and e_poly is model geometry.
w_poly is slow and e_poly is fast. This makes w_poly cause more lag. The difference is, every w_poly could be rendered alone while e_polys are always displayed as a group.
I.e. you have a 6-sided cube (world geometry) but only see 3 sides. Then you have 3 w_poly.
If you see the same cube as a model all 6 sides are calculated because you can see some part of it.
So if a high-poly model or player is in your field of view, then the e_polys increase by 3000 or so, and if there are lots of players there are more e_polys that the engine renders. So e_poly cause lag, only when there are many of them.
gimli posted a wpoly-epoly-ratio some years ago which can be used as reference:
100 wpoly + [x100] 10,000 epolys
200 wpoly + [ x45 ] 9000 epolys
300 wpolys + [ x25 ] 7500 epolys
400 wpolys + [ x15 ] 6000 epolys
500 wpolys + [ x10 ] 5000 epolys
600 wpolys + [ x8 ] 4800 epolys
700 wpolys + [ x6 ] 4200 epolys
800 wpolys + [ x4 ] 3200 epolys
900 wpolys + [ x2 ] 1800 epolys
These values should work fine on every machine. There should not be more than 1000 wpolys + 5000 epolys in a place. More than this will cause lag on nearly every machine when playing in multiplayer.
what really is unclear is the detail 1,2 and 3. I just set them to 2 because according the post of the new vhlt tools, 2 doesn't cut anything.
world cuts world and all detail levels.
detail level 1 cuts detail level 1,2,3,4,... but no world
detail level 2 cuts 2,3,4,... but no world or level 1
detail level 3 cuts 3,4,... but no world or level 1 and 2
and so on
" A large area means that everything visible from that area must be loaded even if it isn't exactly visible by the player."
That is untrue with hint brushes.
I think Rimrook is right. Large areas are "open" areas. In open areas you can see all VIS-leaves. Cutting it into more leaves with HINT makes no difference. You can still see all VIS-leaves.