Hate to sound like a Nike commercial, but "Just do it."
Stock speaker wire = bad, mmkay!
Nothing wrong with overgauging wire, you won't have to redo it if you upgrade later.
Sup tiburon buddy? I have a white 03. I've had the best luck with sub back, port back. I have not tried sub up port back, I hear it works pretty well in the hatches. 2 12s or 1 15 in a proper box will take up almost the whole trunk.