Im pretty sure you have a worn cylinder, get a compression test done.
pressure slips past the piston and vents to the intake manifold via your original pcv. when fitting the BSH ocv fix stage 1 you block the escape route of the excessive pressure and it then stays inside the engineblock restricting the pistons going down.
as long as you keep the original pcv you won't get any noticable decrease in performance of the engine.
Please let me know if i'm wrong og right
I had the exact symptoms on a mate's 05 gti