Any entity will not block VIS as you found out. You need to use a solid brush to block VIS and it must touch the floor, ceiling and walls also.
If you think the engine can see over, around or underneath your wall, then it probably can. Sky textures play havoc with potentially visible brushes. And a solid brush will not always block the engine's idea of what can and can't be seen.
You need to do some experimenting with this. Hint Brushes are a mystery at the best of times, but worth playing with. It all depends what you want to achieve.
Don't worry too much about the epoly reading, the engine just knows there is a model there and is redering it, wether you can see it or not. It is the wpoly count you should be concerned with.