WWR,
If you are referring to the auto-dimming mirrors, there is not a single limit to how dim the mirror will get on any given situation. Rather, it's a scale that's based upon how close the vehicle behind you is, and how bright their lights are as judged by the little sensor on your rearview mirror. I've never seen one of those mirrors go completely dark, but I have seen them get fairly dim if, for example, a big truck is using it's high beams and is only a couple of car lengths behind you. (Of course, if someone in a truck was doing that to me, as rude as driving with your high beams on into someone's rear window is, I'd be inclined to just pull over and let the jerk pass.)
As far as mods for speed go, I assume your real intent is performance upgrades. I'd recommend you look to things like a better air intake system (Vararam, K&N, etc.) and an upgraded exhaust system (Borla, Corso, etc.) You might also try something like a Diablo Predator tuning kit, to adjust your gearing and shifting ratios. There are a whole host of other things you can do, but these mods are fairly basic and will let your Vette breath a bit better, giving you some more power.
-Patrick
