This picture plainly shows what I think are the shortcomings:
First, with the factory PCV in place, you are still drawing oil vapor straight from the crankcase into the intake manifold in off boost conditions, which is the vast majority of the time on a street car. You wouldn't see this in your hose window on the tester because its in a separate part of the PCV system.
Second, when on boost, the crankcase side of the PCV system shifts its vacuum source to the turbo inlet which is what the GS piece uses as a vac source to circulate blowby vapor through its separator. Since the separator sits on top of the oil filler neck, what stops the separator from having oil vapor sucked back up through the filler neck as blowby comes up from above (and it does)?
As a corollary to the above, the factory PCV is imperfect in its closure when IM vac switches over to turbo inlet vac so blowby still gets shunted into the turbo inlet as well as the IM.
Another issue, seen often, is that by routing the head breathers into the same stream as the crankcase removes one the integral factory designs that actually makes sense in a PCV system, keeping the crankcase side of the system and head side separate. This is important because there are times when the head breathers need to flow both ways. That is, vac being pulled on them but also the heads being able draw in "fresh" air from the turbo inlet.
Something else, the separator piece is awful small. Does it have enough surface area to pull out much oil from the copious blowby volumes these motors see?
Unfortunately, attempts to oversimplify the PCV system for a street driven car always result in compromises that are unacceptable, at least IMO. I've tried myself and I just don't think it works well. My 2 pennies.