What you said is true... but 1 high quality 12" will always sound way better than a comparable quality 10". A simple swap will produce what might sound like a reduction in quality, however, the equalizer must be set to compensate for the new range in frequency, etc.
12" > 10" sorry. I used to have 2 10" RF Punches in my last car and in this car decided to go with a single quality 12" competition woofer. Trust me, the single 12" sounds so much better than dualing 10"s