True, but power rating has nothing to do with woofer throw.
two speakers could have similar deign and exact same woofer throw,
while the other might require more juice to move the cone.

Your tenth of a watt headphones are simply much more efficient than standard headphones,
much like those really thin home stereos from Bang & Olfsen.
Efficient speakers + clean amp signal is just one way to arrive at accurate sound.

Now I'm sure that there is a wristwatch out there
that is considered to sound similar to the price of tea in china, but
in a comparison like the one the origional poster will soon be making:
"should I buy the 20watt 5.25" coax or the 50 watt 5.25" coax for the front doors of my car,
The solid advice is that the 50 watt speakers will probably produce a a more accurate sound (at higher volume)
We can do algera and find hypotenuse's 'till the cows come home, the bottom line is that in your car, a set of 50 watt speakers will sound better (more accureate at higher volume) than the 20 watt ones that came with the cear, provided they are adequately powered.