simply put, you are correct, but...
after a certain point, the gains become ireelavant, because you cannot notice the gain. it's like saying an engine mod will give you 0.0001 HP. It won't make you go any faster, it won't even make it feel like you're going any faster, it's only a worthless pain in the ass to do the mod, and also a waste of money. thicker wire is typically less flexible, more expensive, and more dificult to install.
this site will give you a good idea of what wire size you will need:
http://www.eatel.net/~amptech/elecdisc/caraudio.htm
just goto "wire" on the column on the right side, there's lots of places where you just put in the numbers, and it figures everything out for you.
if you don't understand anything, there's an explanation for just about everything somewhere on the site.