You mean up the wire guage? you could either up the voltage or up the current to make more wattage, but in the case of car stereos you would have to up the current at 12v to make more watts along with the various elec. internals. but you're right I retract my last statement now that I think about it. you can only pull so much current through the factory wire harness.
but to answer Jeremy's question ... I would look at the RMS rating of your mids (what did you get btw?) and decide if your feeding them enough power. Your head unit puts out somewhere between 15-25 watts RMS (Tell me what model you have and ill tell you for sure)
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope