'because the vent is now at the bottom of the tank.'
Click. I've just got it. I hadn't considered that when a standard tank is inverted that it now acts like a uniflow tank because the vent is now at the bottom of the tank. I guess that's what you were trying to say all along?
So in my example of the 2" high tank, half full, mounted 1" high the difference in head upright to inverted is uniflow 2", standard 3". So the uniflow is better. Of course if the tank is low it's the other way round, uniflow 2" difference, standard 1". Standard is better.
And yes that explains why manufacturers recommend mounting the tank slightly low.
I'll change my posts a bit.
Dave H