Bruce,

I'm not sure a uniflow will buy you much if you are using pressure.

A uniflow helps compensate for the engine's ability to draw fuel by evening out the pressure seen by the venturi.

George

Edit: There is an easy way to check:

Install a uniflow tank using a standard setup. Try it with the uniflow supplying only atmospheric pressure, then add muffler pressure to the uniflow and try it again.

Next you plug the uniflow vent and open the fill or overflow to atmosphere and try it. Now add muffler pressure to the open tube and try again.

Of course you have to establish your needle setting for each combo so it may take several flights on each to establish how the engine runs at takeoff and just before it quits.

You should now have your answers..without speculation.