I also have the BRD engine, and was getting the low boost fault, including regular visits to limp mode.
Audi claimed I needed a new turbo (at £2500)
It went to a local independent garage, and they sorted it within a couple of hours. What they found, was a dodgy connection on the electrical plug to the actuator, rather than a problem with the actuator itself. They sorted it out properly, and the underboost issues disappeared straight away, and have never returned. that was 2.5 years ago!
I'm only saying this as you never know, it could also be your problem.
Cheers
MMM