Finally - one other person who understands that a pressure tank is NOT really uniflow, even if the pressure input is submerged.
I am not sure what point you are trying to make, but you are probably wrong. It works the same, all you are doing is changing the pressure reference from atmospheric pressure to muffler pressure. The effect is to raise the feed pressure to the muffler pressure, but otherwise it holds the a setting independent of the fuel depth, and to first approximation, holds the mixture constant.
It contains (hypothetically) two additional negative feedback loops.
The first is where the mixture is also controlled by the muffler pressure, which is (imagined to) go up with RPM, in which case the mixture is richened, which (presumably) slows the engine, (possibly) lowering the muffler pressure to go back down. This is all presumptive of the fact that that the muffler pressure goes up as the engine leans out/speeds up. That might happen, but at best the effect is weak, and it is not clear to me that this is what would happen in flight, given that the residual combustion pressure on the firing strokes is MUCH higher in the 4 than in the 2 (which is also why you don't get twice as much power when it goes from 4 to 2).
Ram air into the (open) uniflow vent has the same effect, and is more reliable - and sometimes works too well. Most people prefer to run neutral or slight ram air effects over at least pipe pressure.
The second is where the exhaust heat warms the fuel through the flight, causing the fuel viscosity to go down, slowly making the engine go richer - or suddenly causing it to foam from vibration, which provides positive feedback and an unstable system. When/how this happens seems pretty unpredictable, and only certain cases are reliable in this sense. It also changes radically when your starting fuel temperature not constant - cold days has much more drastic effects, particularly near the end of the tank, when the volume of fuel goes down, it heats up faster, and if it was really cold to start with, you have opened the needle a lot to get the fuel, and now it needs to be closed down again.
Testing is the only way to tell what is going to happen in any specific case. I would greatly prefer to not use muffler pressure because of the sometimes unpredictable behavior, but when it works it definitely has a beneficial effect, uniflow or suction.
Brett