Contractors....HVAC ductwork question
Your reasoning seems sound Steve.
I don't think that the boundary layer is going to play that much role. You are going from a perimeter of 54" to 58", so there may be a little more 'drag' on the airflow, but I think that it will be minimal.
Boundary layer increases as surface area increases. You are only going from a perimeter of 54" to 58". boundary layer is only on the surface of the flow, not the center.
Usually most of your resistance/loss is when you turn bends. Like as in porting heads, straighter is better.