With the same pump flow rate is going to be THE SAME for each tubing diameter in open system test (tubes connected to pump, reservoir and receptacle). Velocity of fluid will vary and be lower with increased diameter.
What I described was an experiment where you can keep VELOCITY constant but flow rates are changing depending on tube diameter.
What you say is right about the point of decreasing performance.
If you looked at Re equation you could see how it is dependant
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I am no avionics expert I am affraid and I cannot say if your analogy is right. If you had your gas as incompressible then yes, it would behave as fluid
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Boundary layer is yet another matter