I have 2 bedrooms that have poor temperature control. The rest of that floor is the main living areas and is very open with great temperature control. I found that I definitely need more insulation in the artic, especially over those two rooms. One of the bidding contractors, like many do, had some other recommendations, which is where I have some questions.

I have a 4 foot plenum and 8 ducts coming out from it. 7 ducts exit the sides and 1 exits the top. He said that the air wants to move laterally so that the top exiting duct will be underserved. This duct goes to one of the rooms that need help. He also thought that 8 ducts in a 4 foot plenum could be improved. He's recommending an 8 foot plenum, with dampers on each duct. I asked whether the larger air box would reduce the pressure and the volume or velocity of the air going through the ducts. He said that it would incease back pressure and would be beneficial to airflow.

1. Is the duct coming out of the top an airflow issue?
2. Is a larger plenum a better solution?
3. Are dampers a benefit?

Thanks in advance!

Air doesn't "want" to move laterally. It wants to go where the pressure is lower, and preferably in the direction it's already going. Your plenum is a pressure filled box, with 8 pathways to lower pressure which should result in airflow. Any outlet at the far end of the plenum will get a bit more due to the directional velocity pressure of the moving air. But it sounds like the air has to turn 90-deg for all 8 outlets. You'll want to try and lower any constriction in the 8 ducts. Dampers add variable constriction that let you fine tune the amount of air in each and preserve the pressure in the plenum.

1. NO
2. YES
3. YES YES, esp with this problem and a bigger supply plenum

You are decribing a spec/tract house system, so
"pay me now or pay me later" applies

Pressure can be both static and dynamic. A plenum by definition is a large enough volume that builds up static pressure and removes any dynamic pressure due to the volume. If large enough and if the box is working truly as a plenum, there is no difference in pressure at any other of the duct ports.

My DIY experience with plenums. Bigger is better. Dampers are worth having but won't solve bigger problems caused by poor duct system design. Don't put a duct outlet on the end of the plenum.

Sounds like he's headed in the right direction. There should be a foot on either end of the plenum that has no ducts connected in that area. Taking a duct out of the top isn't ideal but sometimes space dictates we have to. Never take a duct out of the "lid" as it will take all of the air.
What you did not say was if he thought the sizing of the ducts to those rooms was sufficient? If they are 7" ducts that were not getting full capacity then moving the ducts will help, if they really needed to be 8" then you're still going to be uncomfortable but not as uncomfortable as before.

I think the primary issue is insulation, with airflow being secondary. Across the whole house, the insulation is only 6 to 8 inches deep, which is below what I need (closer to 12 to 15 inches). Over the two bedrooms, there is less insulation than the rest of the house and, in the case of the smaller bedroom, there is no insulation but a single level of insulation board.

The HVAC unit is on the right side of the attic and the two bedrooms are on the left side of the house, so their airflow would be more challenging. I'm willing to spend money on airflow, but needed some input like y'all have provided to make sure that I'm going after the right things.