IMO... the only way to get more power out of it without changing the head unit would be to put in a small inline amp, but on the stock setup would be difficult. Replacing the speakers in the car MIGHT get you a more crisp cleaner sound but wouldn't be any more powerful without an amp boosting it because you still running off the same power from the head-unit
talk with a couple of our Audio Gearheads... I know Black_Friday has his own audio shop, so he would know for sure, try pm'ing him to see if he knows
good luck