If the area of the exhaust doesn't have any effect than why have one at all? Just because a 2" hole came on the drum that was designed to store and transfer fluids does not mean that by coincidence that it is the optimal size to control exhaust on a UDS at any temperature.
The longer pipe only serves to create restriction and restrict air leaving the smoker plain and simple - for all intents and purposes air restriction inside the smoker is the same and any pick up in velocity as compared between 6" and 12" is negligable. By your argument, any restriction on your exhaust will create rancid tasting food no matter what size the opening.
None of the things you mention at such low relative pressures and velocities contributes at all to your explaination of how the thermodynamics of a domed lid vs a flat lid requires different consideration for exhaust sizing.
The results are 4 - 3/4" holes have adequate area to control the exhaust on a UDS. A 2" exhaust will also work although it will require different intake settings as well how much charcoal you initially light up.
To the OP - this is much simpler than is being made out to be. Start out with a 2" hole or 4 - 3/4" holes and see what works for you. Drum lids are cheap.