Hmm, that's actually a pretty significant difference considering the stock system was 360w in it's entirety. You could probably get away with the stock wiring, but if that were me, I'd run 14 gauge, 16 at a minimum. In audio, resistance ain't your friend, and the smaller the wire, the more resistance. The rule of thumb I would go by is if you've got more than 75w RMS to the speaker then you're want to step up to at least 16 gauge. That coupled with you stepping down to 2.5 ohms you're looking at a fair amount of current moving. So i'd do 14 to be sure that wiring doesn't become your bottleneck.