prolly get bashed for this, why not 2 or 3 runs of "0" gauge, cheap 0 gauge is usually like 2 gauge or beefy 4 gauge, recommended doing this with a friend of mine in a lincoln ls, main batt was in the trunk, we ran 3 cheap 0 gauge runs from the alt to the trunk, the place that sells cheap car audio power wire had "0" gauge for less than $2 a foot if you bought more than 15 feet
he runs about 1800 rms his voltage drops to about 12.8-12.9, yes we 2 gauge welding cable to his amps, but your changing lines do have to be able to keep up, again im sure not too many people will like the idea of using 2 or 3 runs of cheaper wire but from how his system is keeping up i would recommend this if money is an issue...