I had similar issue with low boost, but it was happening from time to time. Sometimes when you push the pedal hard the car would lose all boost and would barely move. Did all the things you did and found nothing. Then it turned out it is the rubber connection that goes from the charge pipe to the turbo it has some minor crack and was leaking boost from there. The car was 2007 123d, I know it's not F20, but hopefully this information can help you. I replaced the rubber hose and the car ran like new
