I remember reading about the difference between those three, and the conclusion seemed to be that the difference wasn't really that big. I doubt it's a real issue on my map, unless the difference between NULL and BEVEL has something to do with blocking vis. I base this on the fact that my problem isn't that I have too many faces overall; it's that too many of those I have are being rendered when they shouldn't be.
In case I'm wrong about BEVEl though, and it has to do with the compiler figuring out when to render stuff the way I pointed it out, then please let me know, and I will try using it and SKY instead of NULL
And smellycat, I don't know about most of what you're asking, but about the func_walls: I guess the point is exactly to use that on things that aren't important in any way for vis blocking, like details and small complicated things, again, unimortant for vis. The point of using func_walls is to avoid brushes being cut into smeller bits at intersections.
However, you have to keep in mind that since it doesn't block vis, it all will be rendered, so you want to use it on small complicated stuff, that otherwise cuts up it's surroundings (Remember the 1 unit method as an alternative to func_Wall). A general rule is to never use it on actual walls, ironically.
Also, the way I've come to understand it, the engine just renders the func_walls as epolys instead of wpolys, so don't turn things into func_walls if you don't have to.
So to answer you question: I think the MAIN reason for using NULL is to avoid rendering faces that you can't even see, but I guess also, as you point out, to avoid those faces cutting up things (I'm not really sure if it's actually the edges that cut things and not the faces themselves though?). So no, you won't gain the same if you add your object to a func_wall, as the whole object will be rendered, and that as epoly instead of wpoly. What's better is up for you to decide in the specific situation.
I'm still pretty new to the whole optimization though, but hopefully you will get a reply from someone who actually knows these things for sure.
