Not to mention, Bob, that the uniflow effect works with case pressure just like it does when the uniflow vent is exposed to ambient pressure. The uniflow controls the tank transition.
The benefit of a pressure fuel system with a "standard" vent is that, while the amount of the transition pressure change is still the same, it is a smaller part of the total pressure the engine "sees", hence less overall effect.
The uniflow still operates, so even if if you add the constant head pressure, the transition is (essentially) zero.
Remember the Rat tanks with the pressure vent half way out in the tank, maybe up under the quick fill? Remember the "conventional wisdom" that you needed a 7 or 8 ounce tank so that you didn't get a lean run at the end? Remember that after a 35 (earlier 47) lap segment that the tank was still half-full? Can you say uniflow?
It's all in the physics of the situation (as opposed to "stunt physics")