Uhh, upgrading the stock alt cable will give less resistance which would mean less of a voltage drop. Just because the cable is able to handle the current doesn't mean it isn't corroded to hell and causing a bad voltage drop.
You're correct about the corrosion, this is very possible, but assuming that the cable is in halfway decent shape, then this is really not an issue. the issue is really with the ground straps which are much more prone to corrosion. A voltage drop of even as much as .5v is not going to make a difference in the sound or output. If the system is too weak, it is just too weak. adding the alternator cable is not some sort of miracle cure for electrical systems as its touted by so many people on here.
example.. 13.5v x 100amps = 1350w. 14v x 100amps = 1400w. A gain of 50w in a 1000w + sound system will translate into zero audible gain. to have an audible gain, you would need double the wattage 2000+.
Personally I would just do all of them. You will already be under the hood, and to get a 5ft cable and a fuse and holder isn't that expensive.
I hear that, but Ive seen many posts asking about "do i really need a fuse on my alt cable" and I'd rather see someone omit the alt cable if they don't have a fuse then to put a big cable on without it. to some people $10+ dollars is money better spent else where. and safety is priceless.
think of it this way. you could beef up all of your wires in your car, what would it gain you if they are already sized properly? lower resistance across the board, but it would have no effect on how everything operates.
People will do what they will, I'm just trying to let people know the theory behind things. Im not going to discuss this any further because its really not that big of an issue, just thought Id mention it. It's just not a must have item.