Speaker sensitivity has nothing to do with it, with an 18 gauge wire I can drive almost any speaker behind its intended design. I don't know where you are getting this 30 watt RMS figure from either, it is rather arbitrary and doesn't really mean anything. The concern is the overall resistance the wire itself presents to the amp, and length has A LOT to do with that as does the wire material, number of strands, the purity of the metal, etc. We are talking about 5 feet. It is negligible. if you look inside giant speakers, the woofers often have 18 gauge going from the crossover to the driver. It's a short run.
I don't think the door which Is somewhat exposed to the elements is a great location for xovers either. But... There isn't a whole lot of space in this car. I was trying to get 4 conductors buys I may not even mount the tweets in the doors. I'll see.