Yeah, it has like an 8 psi spring, which is why I think I still think it is your BCS. Think about it like this, if it couldn't move, it wouldn't open and you would have run-away boost. Given a line straight from the compressor to the WG actuator you would have 8 psi (~1/2 boost) because of the spring holding it closed until the pressure in the line was sufficient. You can test that. If you have much less, the WGA spring is too weak, WGA arm not connected properly, or a mechanical issue. Any of the above would contribute to low boost with a good BCS. Leaks would actually increase your boost - that is what your BCS does - it leaks pressure from the line so the WG stays closed longer/more. The restrictor pill also reduces the flow so that the BCS can efficiently leak pressure and as calibrated by the ECU. Your ECU is a factor as it can decide your car is running so crappy that it should pull all boost control, leaving you at WG spring pressure again. That is why CEL or DAM is questioned. Tubing ID and the T must also be correct. These are really the only options other than mech.
Otherwise you are looking at an exhaust restriction, maybe bad UP cat, bad turbo, bad compression, etc.