Let me try to put it in other words.... Excellent sound in a vehicle is full of compromises. Usually, we settle for better sound for the front seat passengers, where people are the majority of time. This dictates mids and highs in the front only... mids and highs in the back will give you no front sound stage... a live performance usually is in front of you and has you looking forward.. this tends to give you the most natural sound.
Many people are brainwashed by the factory speaker placement syndrome... just because car manufacturers would lead you to believe that a speaker at every seat is better does not mean it is so.
Movie theaters do have speakers on the side walls and sometimes the back... but these are sent a dedicated surround signal which has no mono/center stage information. Consequently, while there is a 360 degree sound field, there is still a virtual front stage, where people in front of you on the screen sound like they are in front of you. Putting stereo speakers in he rear of a car, unless they are surround or delayed speakers, is not the way to the best sound.
And if you put a high end component set in the front and lesser coaxes in the rear, your sound will be a blend of the high end and coaxes speakers and sound medeocre... so you may as well put the same cheap coaxes all around if you are going to fill all the holes.
I would just put one good set of components up front... while it won't sound quite as good for the rear passengers, it will still sound good, and the front seat will sound even better.
Best set up would be speakers in the kicks. Second best would be a three-way in the doors, but only if all speakers can be kept as low in the doors as a two-way system.
In the case of a two-way system in the doors, as I said, everything else being equal, a 6X8 has the potential for better midbass, efficiency, and depending on the axis in the doors, could have better or worse off-axis dispersion than 6.5" round.
If you put some very good components up front, add the sub in the rear, with a generous amount of power and a sub am/channel with a high order crossover (18 - 24 db per octave rather than the standard 12), And an Excellent installation, I think you would be amazed. And if you only have room for a single ten or twelve inch sub, don't be afraid to put twice the amount of power or more on the sub as on the front components.... it's easy to turn down the gains if you have too much, but if you don't have enough output to keep up with the front stage... I have seen people that refuse to install an adequate subwoofer system disappointed too many times.