No need for that much wattage to the rear speakers. The main problem is sound direction. The rear sound sucks from the perspective of the front seats.
I have Kappa 6x9s in the rear, running 70W RMS each speaker and they sound great ............ If you are sitting in the rear.
It wouldn't be too much wattage. You don't have to go purchase a high output amplifier for the rears. Having two amps is a little much for someone just throwing in speakers, but its certainly a route that could be taken to gain volume in the rear. Trust me, my rear speakers are dull compared to my front speakers, and my amplifier feeds both front and rear an almost comparable wattage. I have put my ear up to the speaker and it doesn't sound as great as the fronts. And I have had friends in the backseat asking why the rears aren't as loud. But I also have 5-1/4" and you have 6x9's.
But you are damn correct. Their direction is at most peoples feet. And last time I was in biology class, I remember something about someone saying we hear from our ears, not our feet.
So to continue off of what you said about sound direction, I would say a good
counter would be to install a component system in the rear, and fabricate a mounting location for tweeters, preferably higher on the door, near our ears! But that is of course the route for someone who is searching for a competition sound system. This idea would be a typical waste of time for the average listener.
My girlfriends Passat has the same setup front and rear as we do in our fronts. And her rear tweeters are bright, making the rear more appealing. As well, cars just have better sound positioning. They can have 2 6x9's behind the rear seat headrests, and the rear window projects all of the sound forward, towards the passengers, but we don't get that luxury, being in a brick-shaped vehicle
