I see you replied on few threads so I'll just sum it up here.
Quake (or idTech) engine cvars are usually separated in "logical blocks". Prefixes start with common sense labels...
cg_ // client game
ui_ // user inferface (UI + CG form a client based mod if you'll ever go in moding and address (mod) logic)
g_ // game
sv // server (engine)
cl_ // client (engine)
r_ // renderer
s_ // stereo
This are generally all the default cvars you can get with exception of few (/help etc). Most of other cvars are added on top of default game (mods have no limitation or control how they're labelled so it's a wild guess and dev's choice how to name them).
Cvars (settings) itself, have no 'designated' exploits tied to them, but due share amount of them they can and usually do form some sort of exploit. I wont go into particulars but I will reply to your other threads so you get an idea.
1) http://www.rtcwcoop.com/forum/viewtopic.php?id=172
Primitives only work on pseudo "quantum" state. 1 is SP (single pass) and 2 = MP (multi pass). Quantum element is 0, which stands for auto...problem is that 0 (auto) always revert to 1 since choice of auto is either 1 or 2. Auto in this case is broken because it's assumed that "AUTO" would have some sort of a check where, it would establish what's the best option for a computer that is running on....in this case auto is always 1 thus, 0 is same like 1 and as such, renders auto completely useless because it's always 1...
Anything above will kill the renderer...there's simply no "code handling" and no "value handling" for this cvar, so if you set it to 3 or more..game will not know what it is, as it has no error handling and as a result, you will end up with black screen.
2) http://www.rtcwcoop.com/forum/viewtopic.php?id=83
High, low..are just configs that are dumped into game distribution. They're not guides or anything, it's just a dump from testers and QA team, and based on what seemed at that time a reasonable choice...so do not bother with them much because they're simply obsolete.
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As for rest of the cvars or in general.
No, there's no cvar or currently known combination of cvars that would remove the walls. What you can do is remove certain elements of the map..like you can remove tree branches completely - with rmse + picmip etc..as well as other tweaks that can help you a lot in game..like r_depthbits 16 will create a wallhack effect for when you're snipping in mp_beach map and player is on upper bunker- I do not know for when/where it happens in SP/COOP but it's surely there in some form...as well as other problems.
Bottom line is; Yes. cvars can do shit load of a difference, but no, default settings generally only offer mediocre advantage- one that skilled player can always overcome.
If you want to dive into 'cvar world'. I suggest you spent a day or two figuring out what abbreviations stand for and how they show up in game..once you get your foothold, you'll easily handle any unknowns...8 years ago or so I've spent whole day tweaking my (simple) config, but that was time well spent as it given me insight into how things are done.