Plumbing a radiator without a cap?

Done well and done right the expansion tank design is excellent for getting the air out of the coolant and keeping it out. It should have the pressure cap and the cap should be the highest point in the system. When possible most water pumps will benefit from having the bottom of the tank feed the center of the pump impeller. All localized high points should have a small bleed hose or tube that goes to the top of the tank.

The inline cap design is a bad one. The coolant flow acts to reduce the effective pressure below what the cap is rated. To get the pressure that you want you'll have to run a racing type cap with a higher pressure. I can't predict how much higher, you'd have to experiment.