Look for a speaker with a higher "sensitivity" rating. This will allow more volume with the given amount of wattage provided by your stock unit. Nearly any decent aftermarket speaker will have a higher sensitivity rating than those that came stock. Unless you're wanting to drive a massive woofer with your stock unit I wouldn't worry about the power side of things.
I swapped out my stock woofer for a 6" JL(need a new amp though). It was in the way back. To seal it better I put rubber cement and silicon around the inner edge to give it a better sound. That might work for you if you don't want a vibrating panel.