Frank, you're correct about pressure to the overflow, and plugging the uiniflow. Might as well use an old style suction tank with muffler pressure.
I've heard reports of guys having issues with pressure on uniflow tanks. I believe a lot of problems arise from what style of uniflow you use.
A standard metal uniflow tank has the uniflow fuel tank end soldered to the pickup line. This works fine, but are susceptible to several possible issues.
First, you will have to physically raise or lower the tank to adjust the run, upright, and inverted. If you don't have the room to move the tank, you can remove the back, and re-locate the uniflow end.
Second, Pointing the outside uniflow end into the airstream to take advantage of ram air pressure has it's own problems. Going into the wind, speed slows due to the force of the wind. For instance, 55 mph stunt airspeed + 15 mph head wind = 40 mph actual speed relative to the ground. Pressure goes up, and engine richens up, slowing your plane further. The opposite occurs on the downwind leg. This can cause your plane to speed up downwind.
Third, Attaching muffler pressure to the uniflow may be adversely affected by the location of the inside end of the uniflow. It's generally believed that the inside uniflow end should be 1/4" to 1/2" forward of the pickup end. Too close, and air bubbles can be sucked into the pickup. It's not well understood that the inside uniflow end can be much further forward, up to about the middle of the tank, and work as it should, without the chance of introducing air bubbles into the pick up.
Fourth, It mightr be best when using an unpressurized uiniflow tank to place the uniflow opening inside a protected, from varying ram air pressure, area inside the cowl, high enough so it won't siphon out the tank.
My preferences, and personal choice for uniflow tanks is the humble RC style clunk tank. I've used them on every plane, except one, (An Excalibur !, a Dick Mathis design) for the last 25 years. I'll try to locate my tank illustration, and post it here so you can get a better idea of what I do. Thanks to My late friend Allen Brickhaus who clued me into this style of tank.
Over the years, I've streamlined the concept to where it seems to work better. There are some real advantages. Let me list a few.
First, These are easily available, and come in usable sizes.
Second, They are lighter in weight, size for size, than metal tanks.
Third, Set up like mine, they are true clunk tanks. The uniflow is not affected by the movement of the clunk fuel pickup. If the uniflow is tied to the clunk, so it moves with it, The apparent height of the tank is constantly moving,up and down as the clunk moves up and down. Not a good thing and defiantly not running like a true uniflow. It also negates the next advantage.
Fourth, The uniflow inside end is moved to about the middle of the tank. It's designed to allow the apparent tank height to be changed by rotating the uniflow tube. This makes it easy to set the tank height without having to move the entire tank. With the uniflow end away from the pickup, muffler pressure works very well. Engine rpm remains constant up, and downwind.
Carefully study the illustration. This really works well.
John Miller