They're not too bad with head unit power, they use a simple crossover and are pretty efficient with the power given. Of course an amp will provide better performance, but the References perform better than most components off of deck power.
A lot better...i have mine running of the HU and i think they sound find. My originals were blown when i bought the car...but i can't imagine the stock speakers sounding better. do yourself a favor, jump to the kappas, it's worth the extra few dollars.