Being that the return air duct appears undersized, increasing the amount of return available to the blower may help the rooms that are too warm (I assume since Phoenix is the location we're talking about cooling issues, not heating) be more comfortable. Only caveat is if the ducts that supply these rooms are sized correctly, or if they are it's then a matter of were they installed with minimal bending, kinking, etc., all of which can reduce airflow.
I'm not buying "the supply is too close to the door" thing you were told by somebody. That would only be a problem with a really crappy supply grill that caused all the air to drop right by the door and then get sucked out into the hallway return. And if that's the case, get a better supply grill. Builders often install the cheapest, most air restrictive models on the market, which also have poor air throw patterns. Don't blame the location right off. It's actually better to throw the air into the room, toward the outside walls, than to throw it back into the room from near the outside walls.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.