I've thought about replacing the blower motor with a motor I have that may be more efficient and provide more air flow. The motor is used for radio controlled airplanes and has cobalt magnets. I will have to do some testing to see what current it uses at 12 volts loaded. Your looking for more current than the stock motor draws. If that current is to high, you would PWM the current lower.
I think the only thing you will gain is a quicker cabin cool down after your car has been parked in the hot sun.