Power is power. It doesn't matter where it comes from these days. Music is dynamic and the internal amp on a radio doesn't always keep up well with that due to size and heat constraints.
What you want to do when choosing speakers is pick out the highest sensitivity you can if you're worried about output, while using the deck power. Majority of time you end up over driving the internal amp and then you blow speakers because it's putting out a clipped signal or there's a lot of distortion(most people don't notice either).
