x5 e70 40d fault code 4530

[320d]

As per heading. having some issues with this code. first car felt like the smaller turbo was not boosting,then after a week was pulling like a train. now fault remains and low to sluggish boost below 2500rpm. no smoke no noise. so busy with fault finding on vacuum hoses/actuators etc. any one ever heard or seen of actuall small turbo failures on these?
so far i have replaced all the vacuum lines and one of the turbo diverter valves which acted suspect but no joy.